Chemistry FBISE SSC-1 Keybook Chapter 1

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  1. What is chemistry? What are various branches of chemistry?

Ans.     Chemistry:

Chemistry is defined as the science that examines the materials of the universe and changes that these materials undergo

Or

The branch of science which deals with the composition, structure, properties and reaction of matter is called Chemistry.

Branches:

There are eight branches of chemistry.

  1. Physical Chemistry:

The branch of chemistry that deals with laws and theories to understand the structure and changes of matter is called physical chemistry.

  1. Organic chemistry:

The branch of chemistry that deals with substances containing carbon is called organic chemistry.

Some carbon compound such as CO2, CO, metal carbonates, hydrogen carbonates and carbides are studied in inorganic chemistry.

  • Inorganic Chemistry:

The branch of Chemistry that deals with elements and their compounds except organic compounds is called Inorganic Chemistry.

  1. Biochemistry:

The branch of Chemistry that deals with physical and chemical changes that occur in living organisms is called Biochemistry.

  1. Industrial Chemistry:

The branch of Chemistry that deals with the methods and use of technology in the large scale production of useful substances is called Industrial chemistry.

  1. Nuclear Chemistry:

The branch of Chemistry that deals with the changes that occur in atomic nuclei is called Nuclear Chemistry.

  • Environmental Chemistry:

The branch of Chemistry that deals with the chemical and toxic substances that pollute the environment and their adverse effects on human beings is called environmental chemistry.

  • Analytical Chemistry:

The branch of Chemistry that deals with the methods and instruments for determining the composition of matter is called Analytical Chemistry.

 

  1. Vinegar contains 5% acid. Acetic and (CH3COOH) is a colourless liquid that has characteristic vinegar like smell. It is used to flavour food. Identify the various branches of chemistry.
  2. Methods of preparation and study of chemical characteristics of acetic acid.

Ans. Organic Chemistry

  1. The study of chemical reactions that acetic acid undergoes in the bodies of human beings.

Ans. Bio Chemistry

  • The study of its component elements carbon, hydrogen and oxygen.

Ans. Inorganic Chemistry

  1. Use of technology and ways to obtain acetic acid on the large scale.

Ans. Industrial Chemistry

  1. Explanation of its transformation into gaseous state or solid state, applications of laws and theories to understand its structure.

Ans. Physical Chemistry

  1. The method and instruments used to determine its percentage composition, melting point, boiling point etc.

Ans. Analytical Chemistry

  • The study of any adverse effects of this compound or the compounds that are derived from it, on the human.

Ans. Environmental Chemistry

  • The study of the effect of radioactive radiations or neutron on this compound or its component elements.

Ans. Nuclear Chemistry

 

Example 1.1

Identify the branch of chemistry in each of the following cases.

 

  1. Photosynthesis produces glucose and oxygen from carbon dioxide and water in presence of chlorophyll and sunlight.
  2. Plantation helps in overcoming greenhouse effect.

iii.        Haber’s process converts large quantities of hydrogen and nitrogen into ammonia (NH3)

  1. Ammonia is colourless gas which present irritating odour. It is highly soluble in water.
  2. A chemist performed an experiment to check the percentage purity of a sample of glucose (C6H12O6).
  3. An analyst determines that NO2 is responsible for acid rain.

vii.       Chlorofluorocarbon compounds are responsible for the depletion of ozone layer.

viii.      α-particles (He++) when bombard on nitrogen atom, a proton is emitted.

Solution:

  1. Biochemistry, since photosynthesis is a chemical reaction that occur in plants (living organisms).
  2. Environmental chemistry, since greenhouse effect is an environmental problem.
  • Industrial chemistry, since large scale production of any substance is the subject of industrial chemistry.
  1. Inorganic chemistry, since it deals with properties of inorganic compounds.
  2. Analytical chemistry, since it deals with analysis of a compound, whether organic or inorganic in nature.
  3. Environmental chemistry since acid rain is an environmental problem
  • Environmental chemistry, since depletion of ozone layer is environmental problem
  • Nuclear chemistry, since nuclear change can emit protons.

 

Identify the branch of chemistry that is related to the following information:

  1. Hair contains a special class of proteins called keratins, which are present in nails and wool.

Ans.     Bio-Chemistry 

  1. Acetylene is the simplest hydrocarbon that contains carbon-carbon triple bond. Hydrocarbons are the compounds of carbon and hydrogen

Ans.      Organic Chemistry

  • While lead is a pigment used by artists for centuries, the metal Pb is the compound is extracted from its ore, galena (PbS).

Ans.      Industrial Chemistry

  1. Sulphuric and (H2SO4) is weaker than hydrochloric acid

Ans.     Analytical Chemistry

  1. Gases can be compressed by applying pressure.

Ans.      Physical Chemistry

  1. Some examples of complete proteins food are meat, milk and eggs.

Ans.      Bio-Chemistry

  • Element radium decays by emitting α-particles and is converted into another element radon.

Ans.      Nuclear Chemistry

  • Calorimeter is a device that measures the amount of heat, a substance absorbs on heating or emits on cooling.

Ans.      Analytical Chemistry

 

  1. Give postulates of Dalton’s atomic theory?

Ans.     Dalton’s Atomic Theory:

Main postulates of Dalton atomic theory are as follows:

  1. All elements are composed of tiny individuals particles called atoms.
  2. Atoms of a particular element are identical. They have same mass and same volume.
  • During chemical reaction atoms combine or separate or re-arrange. They combine in simple ratios.
  1. Atoms can neither be created nor destroyed.

 

Q         Define following:

(i)         Element:

A substance that cannot be converted to other simpler substance is called an element.

(ii)        Compound:

A compound is a pure substance that consists of two or more elements held together in fixed proportions by natural forces called chemical bonds. Compounds have sharp melting point.

(iii)       Mixture:

An impure substance that contains two or more pure substances that retain their individual chemical properties is called a mixture. Mixtures do not have sharp melting point.

  • Atomic Number:

The number of protons in the nucleus of an atom is known as its atomic number.

  • Mass Number:

The total number of protons and neutrons in an atom is known as its mass number

Example 1.2

            Atomic number of an element is 17 and mass number is 35. How many protons and neutrons are in the nucleus of an atom of this element?

Solution:

Number of protons = atomic number = 17

Number of neutrons = mass number – atomic number

= 35 – 17 = 18

 

Write the empirical formulas for the compound containing carbon to hydrogen in the following ratios

  • 1 : 4 CH4
  • 2 : 6 CH3
  • 2 : 2 CH
  • 6 : 6 CH
  1. Aspirin is used as a mild pain killer. There are nine carbon atoms, eight hydrogen atoms and four oxygen atoms, in this compound. Write its empirical and molecular formulas.

Ans.     Molecular formula of Aspirin is C9H8O4

Empirical formula of Aspirin is C9H8O4

 

  1. Vinegar is 5% acetic acid. This contains 2 carbon atoms, four hydrogen atoms and 2 oxygen atoms. Write its empirical and molecular formulas.

Ans.     Molecular formula of Vinegar is C2H4O2

Empirical formula of Vinegar is CH2O

  1. Caffeine (C8H10N4O2) is found in tea and coffee. Write the empirical formula for caffeine.

Ans.     C4H5N2O

 

  1. Define molecular mass?

Ans.     Molecular mass is the sum of atomic masses of all the atoms present in the molecule.

The term molecular mass is used for molecular compounds.

Example 1.3

 

  1. Determine the molecular mass of glucose C 6H12O6 which is also known as blood sugar.

Solution:

Molecular mass of C6H12O6 = 6×12 + 12×1 + 6×16

= 180 amu

 

  1. Determine the molecular mass of naphthalene C10H8, which is used in mothballs.

Solution:

Molecular mass of C10H8 = 12×10 + 1×8 = 128 amu

 

  1. Define formula mass?

The sum of the atomic masses of all the atoms in the formula unit of a substance is called formula mass. The term formula mass is used for ionic compounds. (Having cations and anions)

Example 1.4

 

  1. Sodium chloride, also called as table salt is used to flavor food, preserve meat, and in the preparation of large number of compounds. Determine its formula mass.

Solution:

Formula mass of NaCl = 1×23 + 1×35.5 = 58.5 amu

 

  1. Milk of magnesia which contains Mg(OH) 2, is used to treat acidity. Determine its formula mass.

Solution:

Formula mass of Mg(OH) 2,     = 24 + 2(16+1)

= 24 + 34

= 58 amu

 

  1. Potassium Chlorate (KClO3) is used commonly for the laboratory preparation of oxygen gas. Calculate its formula mass.

Ans.     Formula mass of KClO3           =39 + 35.5 + 3×16

= 39 + 35.5 + 48

= 122.5 amu

  1. When baking soda, NaHCO3 is heated it releases carbon dioxide, which is responsible for the rising of cookies and bread. Determine the formula mass of baking soda and carbon dioxide.

Ans.     Formula mass of NaHCO3       =23+1+12+3×16

= 23+1+12+48

= 84 amu

Formula mass of CO2 = 12+2×16

= 12+32

= 44 amu

 

  1. Following compounds are used as fertilizers. Determine their formula masses.

(i)         Urea, (NH2) 2 CO

Formula mass of  (NH2) 2 CO  = 2×14 + 2×2 + 12 + 16

= 28 + 4 + 12 + 16

= 60 amu

  • Ammonium nitrate, NH4NO3

Formula mass of NH4NO3       = 14 + 1×14 + 14 + 3×16

= 14 + 4 +14 + 48

= 80 amu

  1. Why Na+ has +1 charge?

Ans.     In a sodium (Na) nucleus there are 12 neutrons and 11 protons. So charge on nucleus is +11. Around the nucleus, in the ion there are 10 electrons with total charge -10. Thus the charge on the ion is +11 +(-10) = +1

 

Explain why?

  1. An oxide ion has -2 charge

Ans.     In an oxygen nucleus there are 8 neutrons and 8 protons. So charge on nucleus is +8. Around the nucleus, in the ion there are 10 electrons with total charge -10. Thus the charge on the ion is +8 + (-10) = -2.

 

  1. Magnesium ion, Mg+2 has +2 charge

Ans.     In a magnesium nucleus there are 12 neutrons and 12 protons. So charge on nucleus is +12. Around the nucleus, in the ion there are 10 electrons with total charge -10. Thus the charge on the ion is +12 + (-10) = +2.

 

  1. Sulphide ion, S-2 has -2 charge

Ans.     In a sulphur nucleus there are 16 neutrons and 16 protons. So charge on nucleus is +16. Around the nucleus, in the ion there are 18 electrons with total charge -18. Thus the charge on the ion is +16 + (-18) = -2.

 

            Identify ion, molecular ions and free radicals from the following species.

CN, N, He+2, N-3 , ,

Ans.     Ions: N-3, He+2

Molecular Ion : CN

Free radicals:  , , N

  1. How can we identify particles of elements and compounds?

Ans.     Elements have atoms of same size and compounds have different.

 

Example 1.5

 

Figure shows some molecules. Identify particles of elements and compounds.

           

Solution:

Particles of elements are A,C,D and  E. Particles of compounds are Band F.

 

  1. Differentiate between monoatomic and polyatomic molecules.

Monoatomic:

A molecule that contains only one atom is called monoatomic.

For example:  He, Ne, Ar, Kr, Rn etc.

Polyatomic:

Molecules that contain two or more similar or different atoms are called polyatomic molecules

For example:  H2, O2, HCl, NH3 etc.

 

  1. Observe the given figures and identify the diagrams that represents the particles of:-

                       

(a)        An element whose particles are molecules

(vi) – Radon

(b)        An element whose particles are molecules

(iv) – Iodine

(c)        A compound

(i)  Hydrogen chloride

(d)        A mixture of an element and a compound

(ii)  oxygen and sulphur dioxide

(e)        A mixture of two elements

(v) Argon and Chlorine

(f)        A mixture of two compound.

(iii) Ammonia and Sulphur trioxide

 

  1. Observe the given figure and decide which diagram represents particles in an element, a compound or a mixture.

Particles in:

An element:  (b), (f)

A compound: (a),  (c)

A mixture: (b), (e)

 

  1. Write use of mole concept in our daily life?

Ans.     Mole concept is used to counting a coin. When counting a pile of coins, it would not be convenient to count them one by one. Mole represents definite amount of a substance. As 6.02×1023 carbon atoms weigh 12g, 6.02×1023 coins also have definite mass. By which we count coins easily.

 

  1. Calculate molecular mass in grams

(i)         C6H12O6

Molecular mass of C6H12O6     = 6×12+12×1+16×6

= 180.096 amu

Molecular mass of C6H12O6 in grams= 180.096g.

 

(ii)        H2O

Molecular mass of H2O           =          2×1.008 + 16

=          18.016 amu

Molecular mass of H2O in grams = 18.016g

 

  1. Calculate the molar masses of

(a)        NA

1 mole of Na   =          23g

(b)        Nitrogen

Molecular mass of |Nitrogen(N2) = 14×2

= 28 amu

= 28 g

(c)        Sucrose C12H22O11

Molecular mass of  C12H22O11  = 12×12+1×22+16×11

= 144+22+176

= 342 g

 

            Calculate the one mole of

(a)        Copper

1 mole of Copper (Cu) =          63.5 g

(b)        Iodine

Molecular mass of Iodine (I2) =           2×127

1 mole of I2     = 254 g

(c)        Potassium       

1 mole of K     = 39g

(d)        Molecular mass of oxygen (O2)          = 2.16 

1 mole of O2    =          32

 

Example. 1.5 (a)

 

  1. Oxygen is converted to Ozone (O3) during thunder storms. Calculate the mass of Ozone if 9.05 moles of Ozone is formed in a storm?

Solution:

Number of moles of Ozone (O3)         = 9.05 moles

Molar mass of Ozone (O3)       = 3×16 = 48

Mass (in grams)                       =          ?

Mass in grams = Number of moles x Molar mass

= 9.05×48

= 434.4 g

Example. 1.6

 

  1. When natural gas burns CO2 is formed. If 0.25 moles of CO2 is formed, what mass of CO2 is produced?

            Solution:

Number of moles of CO2         = 0.25 moles

Molar mass of CO2                  = 12+2×16

= 44g

Mass (in grams)           = ?

Mass in grams = Number of moles x Molar mass

= 0.25 x 44

= 11g

 

Example. 1.7

Q         How many moles of each of the following substance are present?

(a)        A balloon filled with 5g of hydrogen.

Solution:

Mass of Hydrogen (H2)                        = 5g

Molar mass of Hydrogen (H2)  = 1.008×2

= 2.016g

Number of moles         =  ?

Number of moles         =  Mass in grams

Molar mass

=  _5___

2.016

= 2.48 moles

(b)        A block of ice that weighs 100g.

Solution:

Mass of (H2O)             = 100g

Molar mass of (H2)      = 2×1.008+16

= 18.016g

Number of moles         =  ?

Number of moles         =  Mass in grams

Molar mass

= _100_

18.016

= 5.55 moles

  1. The molecular formula of a compound used for bleaching hair is Hydrogen Peroxide (H2O2). Calculate

 

  • Mass of this compound that would contain 2.5 moles.

Solution:

Number of moles         =  2.5 moles

Molar mass of (H2O2) = 2×1+2×16

= 34g

Mass (in grams)           = ?

Mass in grams = Number of moles x Molar mass

= 2.5 x 34g

= 85g

 

(b)        Number of moles of this compound that would exactly weight 30 g.

Solution:

Mass of (H2O2)                        = 30g

Molar mass of (H2O2)  = 34g

Number of moles         =  ?

Number of moles         =  Mass in grams

Molar mass

= 30

34

= 0.88 moles

  1. A spoon of table salt NaCl contains 12.5 grams of this salt. Calculate the number of moles it contains.

Solution:

Mass of Sodium Chloride (NaCl) = 12.5g

Molar mass of NaCl                 = 23+35.5

= 58.5g

Number of moles         =  ?

Number of moles         =  Mass in grams

Molar mass

= 12.5

58.5

= 0.21 moles

 

  1. Before the digestive systems x-rayed, people are required to swallow suspensions of barium sulphate (BaSO4). Calculate mass of one mole of BaSO4

Solution:

Mass of 1 mole of BaSO4 = 137+32+4×16

= 137+32+64

= 233g

Example 1.8

  1. Zn is a silvery metal that is used to galvanize steel to prevent corrosion. How many atoms are there in 1.25 moles of Zn?

Solution:

Number of moles         =         1.25 moles

Avogadro’s number =NA=       6.022×1023

Number of atoms         = ?

Number of atoms         = Number of moles x NA

=  1.25 x 6.022 x 1023

=  7.53 x 1023 atoms

 

  1. A thin rod of Aluminum (Al) is used as wrapper in food industries. How many atoms are present in a foil that contains 0.2 moles of aluminum?

Solution:

Number of moles         = 0.2 moles.

Avogadro’s number = NA = 6.022×1023

Number of atoms         = ?

Number of atoms         = Number of Moles x NA

= 0.2×6.022×1023

= 1.2044 x 1023 atoms

 

 

 

 

 

Example. 1.9

 

  1. Methane (CH4) is the major component of natural gas. How many molecules are present in 0.5 moles of a pure sample of methane?

Solution:

Number of moles         = 0.5 moles.

Avogadro’s number = NA = 6.022×1023

Number of molecules  = ?

Number of molecules  = Number of Moles x NA

= 0.5×6.022×1023

= 3. 011 x 1023 molecules

 

  1. At high temperature hydrogen sulphide (H2S) gas given off by a volcano is oxidized by air to sulphur dioxide (SO2). Sulphur dioxide reacts with water to form acid rain. How many molecules are there in 0.25 moles of SO2.

Solution:

Number of moles         = 0.25 moles.

Avogadro’s number = NA = 6.022×1023

Number of molecules  = ?

Number of molecules  = Number of Moles x NA

= 0.25×6.022×1023

= 1.5055 x 1023 molecules

Example 1.10

Titanium is corrosion resistant metal that is used in rockets, aircrafts and jet engines. Calculate the number of moles of this metal in a sample containing 3.011×1023 Ti-atoms

Solution:

Number of atoms         = 3.011×1023 atoms

Avogadro’s number = NA = 6.022×1023

Number of moles         = ?

Number of moles         = Number of atoms

NA

= 3.011×1023

= 6.022x 1023

= 0.5 moles

 

 

Example 1.11

Formaldehyde is used to preserve dead animals. Its molecular formula is CH2O. Calculate the number of moles that would contain 3.011×1022 molecules of this compound.

Solution:

Number of molecules = 3.011×1022 molecules

Avogadro’s number = NA = 6.022×1023

Number of moles         = ?

Number of moles         = Number of atoms

NA

= 3.011×1022

= 6.022x 1023

= 0.05 moles

  1. Aspirin is compound that contains carbon, hydrogen and oxygen. It is used as a painkiller. An aspirin tablet contains 1.25 x 1030 How many moles of this compound are present in the tablet?

Solution:

Number of molecules = 1.25×1030 molecules

Avogadro’s number = NA = 6.022×1023

Number of moles         = ?

Number of moles         = Number of atoms

NA

= _1.25×1030

= 6.022×1023

= 2.076 x 106 moles

 

  1. A method used to prevent rusting in ships and underground pipelines involves connecting the iron to a block of a more active metal such as magnesium. This method is called cathodic protection. How many moles of magnesium are present in 1 billion (1 x 109) atoms of magnesium.

Solution:

Number of atoms         = 1×109 atoms

Avogadro’s number = NA = 6.022×1023

Number of moles         = ?

Number of moles         = Number of atoms

NA

= __1×109___

= 6.022x 1023

= 1.66 x10-15 moles

 

Review Questions

 

Q.1      Encircle the correct answer:-

  • Which of the following lists contains only elements?
  1. Air, water, oxygen
  2. Hydrogen, oxygen, brass
  3. Air, water, fire, earth
  4. Calcium, sulphur, carbon
  • The diagrams below represent particles in four substances, which box

represent the particles in nitrogen

  1.              c.              d.

 

  • What is the formula mass of CuSO4.5H2 (Atomic masses: Cu-3.5, S=32,H= 1 )
  1. 5
  2. 5
  3. 5
  4. 5
  • A compound with chemical formula Na2CX3 has formula mass 106amu: Atomic mass of the element X is
a.106
b.23
c.12
  1. 16
  • How many moles of molecules are there in 16g oxygen?
  1. 1
  2. 5
  3. 1
  4. 05
  • What is the mass of 4 moles of hydrogen gas?
  1. 8.064g
  2. 4.032g
  3. 1g
  4. 1.008g
  • What is the mass of carbon present in 44g of carbon dioxide?
  1. 12g
  2. 6g
  3. 24g
  4. 44g
  • The electron configuration of an clement is 1s22s2. An atom of this element will form an ion that will have charge.
  1. +1
  2. +2
  3. +3
  4. -1
  • Which term is the same for one mole of oxygen and one mole of water?
  1. volume
  2. mass
  3. atoms
  4. molecules
  • If one mole of carbon contains x atoms, what is the number of atoms contained in 12g of Mg.
  1. x
  2. 5x
  3. 2x
  4. 5x

Answers:

i) Dii) Aiii) Civ) Dv) B
vi) Avii) Aviii) Bix) Dx) B

 

 

Q.2      Give short answers:

  1. i) Differentiate between ion and free radical
IonFree radical
i)Ion is a charged specie formed from an atom or chemically bonded group of atoms by adding or removing electrons.Free radical is an atom or group of atoms that contains an unpaired electron
ii)They exist in solution or in crystal lattice.They can exist in solution as well as in air.
iii)Ion may have positive or negative charge.Free radical bear no charge.
iv)It has even number of electrons.It has odd number of electrons.
v)Example

Sodium lose one electron and become Na+ ion,

Chlorine gain one electron and become Cl ion.

 ,

 

  1. ii) What do you know about corpuscular nature of matter?

Ans.     Ancient Greek philosopher believed that everything is made up of four things. (i) Earth (ii) Air  (iii) Water (iv) Fire. The Greek concept of four elements existed for more than two thousand years. In 19th century John Dalton proposed an atomic theory. In 20th century concept of complex internal structure was introduced.

 

iii)        Differentiate between analytical chemistry and environmental chemistry

Ans.

Analytical ChemistryEnvironmental Chemistry
The branch of chemistry that deals with the methods and instruments for determining the composition of matter is called analytical chemistryThe branch of chemistry that deals with the chemicals and toxic substances that pollute the environment and their adverse effects on human beings is called environmental chemistry
OROR
Analytical chemistry is the branch of chemistry that deals with separation and analysis of a sample to identify its components.It is the branch of chemistry in which we study about components of the environment and the effects of human activities on the environment.

 

  1. iv) What is mole?

Ans.     A mole is an amount of a substance that contains 6.022×1023 particles of that substance.

 

  1. v) Differentiate between empirical formula and molecular formula.
Empirical FormulaMolecular Formla
Which gives the simplest whole number ratio of atoms of each element of a compoundWhich shows the actual number of atoms of each element present in a compound.
It does not show the structure of compoundIt shows the structure of compound
Two or more compounds can have same empirical formulaTwo or more compounds cannot have same molecular formula.
It represents ionic and covalent compounds

E.F = M. F

It represents covalent compounds only

M.F = n x E. F

Example
CH2O, CH are empirical formula of glucose and benzeneC6H12O6, C6 H6 are molecular formula of glucose and benzene

 

  1. vi) What is the number of molecules in 9.0g of steam?

Solution:

Mass in grams = 9.0g

Molar mass of steam (H2O) = 2 x 1 + 16 = 18g

Avogadro’s number = NA = 6.022×1023

Number of molecules = ?

Number of molecules  =          Mass in grams x NA

Molar Mass

=           6.022×1023

=          0.5 x 6.022 x 1023

=          3.011 x 1023 Molecules

 

vii)       What are the molar masses of Uranium-238 and

Uranium-235?

Ans.     Molar mass      =          Atomic mass

Molar mass of uranium-238    =          238g

Molar mass of uranium-235    =          235g

 

viii)      Why one mole of hydrogen molecules and one mole of H atoms have different mases?

Ans.     One mole of hydrogen molecule (H2) contain two hydrogen atom, its molar mass is 1×2=2g. One mole of H-atom contain only one hydrogen atom, its molar mass is 1×1=1g. That’s why one mole of hydrogen molecules and one mole of H-atoms have different masses.

 

Q.3      Define:

Ion:

Ion is a charged specie formed from an atom or chemically bonded group of atoms by adding or removing electrons.

Ion may have positive or negative charge.

Molecular Ion:

When a molecule losses or gains electrons, the resulting species is called molecular ion. These are short lived species and only exist at high temperature. Molecular ions do not form ionic compounds.

Formula Unit:-

The simplest unit which represents an ionic compound is called formula unit.

Free radical:-

Free radical is an atom or group of atoms that contains an unpaired electron. Free radical bear no charge

            Example                       ,

Atomic number:-

The number of protons in the nucleus of an atom is known as atomic number.

Mass number:-

The total number of protons and neutrons in an atom is known as its mass number.

Atomic mass unit:-

One atomic mass unit (amu) is defined as a mass exactly equal to one twelfth the mass of one C-12 atom.

Mass of one C-12 atom = 12 amu

1 amu  =          Mass of one C-12 atom

12

Q.4      Differentiate between

(a) Atom and Ion

AtomIon
Atom is the smallest particle of an element that cannot exist in free stateIon is a charged species formed from an atom or chemically bonded group of atoms by adding or removing electrons
Atom is electrically neutralIon may have positive or negative charge.
Example

H, Na

 

Na+, H+

 

(b)        Molecular ion and free radical

Molecular ionFree radical
When a molecules loses or gains electrons, resulting species called molecular ionFree radical is an atom or group of atoms that contains an unpaired electron
Molecular ion may have positive or negative chargeFree radical bear no charge
Example
When O2 losses one electron, it form O2+ and when absorbs an electrons it forms O2ion ,

 

Q.5      Describe how Avogadro’s number is related to a mole of any substance.

Ans.     Avogadro’s number is related to a mole of any substance by a relation.

No. of moles of a substance= No. of molecules of substance

Avogadro’s number

= No. of molecules of a substance

6.02 x 1023

 

Q.6      Calculate the number of moles of each substance in samples with the following masses.

 

(a)        2.4g of He:

Mass of Helium (He)   =          2.4g

Molar mass of Helium(He)=    4g

Number of moles         =          2.4

4

=          0.6 moles

 

(b)        250 mg of Carbon:

Mass of carbon (C)      =          250mg = 250g/1000= 0.25g

Molar mass of carbon =          12g

Number of moles         =          0.25

12

=          0.021 moles

 

(c)        15g of sodium chloride:

Mass of sodium chloride (NaCl)          =  15g

Molar mass of Sodium Chloride          = 23 + 35.5 = 58.5g

Number of moles         =  15_

58.5

=  0.256 moles

(d)        40 g of Sulphur:

Mass of Sulphur (S)     =          40 g

Molar mass of Sulphur =          32g

Number of moles         =          40

32

=          1.25 moles

(e)        1.5 kg of MgO:                                   1kg = 1000 g

Mass of MgO   =          1.5 Kg  = 1.5 x 1000 = 1500 g

Molar mass of MgO     =          24 + 16 = 40g

Number of moles         =          1500    = 37.5 moles

40

 

Q.7      Calculate the mass in grams of each of the following samples:

(a)        1.2 moles of K

Number of moles of Potassium (K) = 1.2 moles

Molar mass of Potassium         =          39g

Mass in grams = 1.2 x 39 = 46.8 g

 

(b)        75 moles of H2

Number of moles of hydrogen(H2)  = 75 moles

Molar mass of hydrogen(H2)    =          2 x 1 = 2g

Mass in grams = 75 x 2 = 150g

 

(c)        0.25 moles of steam

Number of moles of steam(H2O)  =     0.25 moles

Molar mass of steam(H2O)       =          2 x 1+16 = 18g

Mass in grams = 0.25 x 18 = 4.5g

 

(d)        1.05 moles of CuSO4 . 5H2O

Number of moles of Copper Sulphate Penta Hydrate= 1.05 moles

Molar mass of CuSo4 .5H2O = 63.5+32+4×16+5(2×1+16)

= 63.5 +32 +64 +5(18)

= 249.5g

Mass in grams       = 1.05 x 249.5 = 261.96g

(e)        0.15 moles of H2SO4

Number of moles of Sulphuric Acid (H2SO4)= 0.15 moles

Molar mass of H2SO4   =          2 x 1 + 32 + 4×16 = 98g

Mass in grams = 0.15 x 98 = 14.7g

 

Q.8      Calculate the number of molecules present in each of the following samples

 

(a)        Number of moles        = 2.5 moles

Number of molecules  = 2.5 x 6.022 x 1023

= 1.505 x 1024 molecules

 

(b)        3.4 moles of ammonia, NH3

Number of moles         = 3.4 moles

Number of molecules  = 3.4 x 6.022 x 1023

= 2.05 x 1024 molecules

(c)        1.09 moles of benzene, C6H6

Number of moles         = 1.09 moles

Number of molecules  = 1.09 x 6.022 x 1023

= 6.56 x 1023 molecules

 

(d)        0.01 moles of acetic acid, CH3COOH

Number of moles         = 0.01 moles

Number of molecules  = 0.01 x 6.022 x 1023

= 6.02 x 1021 molecules

 

Q.9      Decide whether or not each of the following is an example of empirical formula

(a)        Al2 Cl6                                                 

No

(b)        Hg2 Cl2

                            No

(c)        NaCl

Yes

(d)        C2H6O

Yes

 

Q.10    TNT or trinitrotoluene is an explosive compound used in bombs. It contains 7 C-atoms, 6 H-atoms, 5 N-atoms and 6-O atoms. Write its empirical formula

Ans.     Empirical formula = C7 H6 N5 O6

 

Q.11    A molecule contains four Phosphorus atoms and ten Oxygen atoms. Write the empirical formula of this compound. Also determine the molar mass of this molecule

Ans.      Molecular formula      =          P4O10

Empirical formula        =          P2O5

Molar mass of P4O10    =          4×31 + 10×16

=          124 + 160        = 284g

 

Q.12    Indigo (C16H10N2O2) the dye used to colour blue jeans is derived from a compound known as indoxyle (C8H7ON). Calculate the molar masses of these compounds. Also write their empirical formulas

 

Ans.     Indigo (C16H10N2O2)

Molar mass      =          14×12 + 10×1 + 2×14 + 2×16

=          168 + 10 + 28 + 32

=          238g

Empirical formula =     C8H5NO

 

Indoxyle (C8H7ON)

Molar mass      =          8 x12 + 7×1 + 16 + 14

=          96 + 7 + 16 + 14

=          133g

Empirical formula =     C8H7ON

 

Q.13    Identify the substance that has formula mass of 133.5 amu

(a)        MgCl2

Formula mass  =          24 + 2×35.5

=          24 + 71

=          95 amu

(b)        S2Cl2

Formula mass  =          2×32 + 2×35.5

=          64 + 71

=          135 amu

(c)        BCl3

Formula mass  =          11 + 3×35.5

=          11 + 106.5

=          117.5 amu

 

 

(d)        AlCl3

Formula mass  =          27 + 3×35.5

=          27 + 106.5

=          133.5 amu

 

Q.14    Calculate the number of atoms in each of the following samples.

Number of Atoms = Number of moles x NA

Number of moles =  Mass in grams

Molar mass

 

(a)        3.4 moles of nitrogen atoms

Number of moles         =          3.4 moles

Number of atoms         =          No. of moles x NA

=          3.4 x 6.022×1023

                                                =             2.05 x 1024 atoms

(b)        23g of Na

Mass in grams              =          23 g

Molar mass of Sodium (Na)=   23 g

Number of atoms         =          Mass in grams  x NA

Molar mass

=          23 x 6.022 x 1023

23

=          6.022 x 1023

(c)        5 g of H atoms

Mass in grams              =          5g

Molar mass of hydrogen(H)=   1g

Number of atoms         =          Mass in grams  x NA

Molar mass

=          5 x 6.022 x 1023

1

=          3.01 x 1024 atoms

Q.15    Calculate the mass of the following

 

Mass in grams  =          Number of atoms x  Moles mass

NA

(a)        3.24 x 1018 atoms of iron

Number of atoms         = 3.24 x 1018 atoms

Mass in gram   =          3.24 x 1018 x 56

6.022 x 1023

=          3.01 x 10-4 g

(b)        2 x 1010 molecules of nitrogen gas

Number of molecules =          2 x 1010 molecules

Molar massof nitrogen gas (N2) =  2 x 14 = 28g

Mass in gram   =          Number of molecules x Molar mass

NA

=          2 x 1010 x 23

6.022 x 1023

=          9.3 x 10-13 g

(c)        1 x 10 25 molecules water

Number of molecules  =          1 x 1025 atoms

Molar mass of water (H2O)=    1×2 + 16 = 18g

Mass in grams              =          1 x 1025 x 18

  • 1023

=          2.99 x 102g

(d)        3 x 106 atoms of Al

Number of atoms         =          3 x 106 atoms

Molar mass of Aluminum(Al)=            27 g

Mass in grams              =          3 x 106 x 27

6.022 x 1023

=          1.346 x 10-16g

Q.16    Identify the branch of Chemistry that deals with the following examples

  1. A cornstalk grows from a seed

Ans.     Bio Chemistry

 

  1. Dynamite (C3 H5 N3 O9) explodes to form a mixture of gases

Ans.     Inorganic chemistry

 

  1. Purple iodine vapour appears when solid iodine is warmed

Ans.     Analytical chemistry

 

  1. Gasoline (a mixture of hydro carbons) fumes are ignited in an auto mobile engine

Ans.     Organic chemistry

 

  1. A silver article tarnishes in air

Ans      Environmental chemistry

 

  1. Ice floats on water

Ans.     Physical chemistry

 

  1. Sulphur dioxide is the major source of acid rain

Ans.     Environmental chemistry

 

  1. Many other light chlorinated hydrocarbons in drinking water are carcinogens

Ans      Environmental chemistry

 

  1. In Pakistan most of the factories use wet process for the production of cement.

Ans.     Industrial chemistry

 

  1. Carbon-14 is continuously produced in the atmosphere when high energy neutrons from space collide with nitrogen-14

Ans.     Nuclear chemistry.

 

 

 

ThinkTank

 

  1. What mass of sodium metal contains the same number of atoms as 12.00g of Carbon

 

Ans.     Avogadro’s Number = NA = 6.022×1023

 

1 mole of Sodium atoms = 1 mole of Carbon-12

 

= 6.022×1023 molecules

Therefore 23 g of sodium metal contains the same number of atoms as 12.0 g of carbon

 

  1. What mass of oxygen contains the same number of molecules as 42g of nitrogen?

Solution:

1 mole of N = 14g

mole of N = 1g

42 ×   mole of N = 42 × 1g

3 moles of N = 42g

 

Similarly,

 

1 mole of O = 16g

3 × 1 mole of O = 3×16g

3 moles of O = 48g

 

Therefore 48 g of oxygen contains the same number of atoms as 42g of nitrogen

 

  1. Calculate the mass of one hydrogen atom in grams.

 

Ans.     Mass of 1 hydrogen atom =  ____Atomic Mass__

Avogadro’s Number

 

=   __1.008__

6.022×1023

 

Mass of hydrogen atom = 1.674×10-24g

  1. Observe the given figure. It shows particles in a sample of air.

 

  1. Count the substances shown in the sample

Ans.     There are five substances. These are N2, H2O, O2, CO2 and Ar.

  1. Is air a mixture or pure substance? Explain?

Ans.     Air is a mixture. because air is composed of different gases

  1. Identify the formula of each substance in air.

Ans.     N2, H2O, O2, CO2 and Ar.

  1. Decide whether each substance in air is an clement or a compound.

Ans.     There are elements as well as compound in air.

Elements: Nitrogen, Argon and Oxygen

Compounds: Water and Carbon di Oxide.

  1. What is the most common substance in air?

Ans.     Nitrogen is the most common substance in air.

  1. Calculate the number of H-atoms present in I 8g H2

Ans.      1 mole of H2O = 6.022×1023 atoms

Since there are 2 moles of hydrogen in water, so:

 

Number of hydrogen atoms = 2×6.022×1023 atoms

 

= 1.204×1024 atoms

  1. Calculate the total number of atoms present in I8g H2

Ans.      1 mole of H2O = 6.022×1023 atoms

Since there are 2 hydrogen atoms and 1 mole of oxygen atom, so, total number of atoms are 2+1=3

Total number of atoms = 3×6.022×1023 atoms

= 1.806×1024 atoms

15 Responses

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  7. Simam says:

    Can anyone tell why the following differences are not included in differentiating molecular ions and free radicals:
    1-molecular ions do not have unapaired electrons while free radicals have an unpaired electron
    1-molecular ions have even number of electrons while free radicals have odd number of electrons

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