# 3 - Atoms and Molecules

PAGE NO – 32

Q1  - In a reaction, 5.3 of sodium carbonate reacted with 6g of ethanoic acid. The products were 2.2g of carbon dioxide, 0.9g water and 8.2g of sodium ethanoate. Show that these observation are in agreement with the law of conservation of mass. Sodium carbonate + ethanoic acid    -      Sodium ethanoate + carbon dioxide + water
Ans – According to the law of conservation of mass :
Mass of reactants = Mass of products
Mass of reactants = mass of sodium carbonate + mass of ethanoic acid.
= 5.3 g +6 g
= 11.3 g
Mass of product = mass of sodium ethanoate + mass of carbon dioxide + water.
= 8.20g + 2.2g + 0.9 g
= 11.3 g

Q2 – Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3g of hydrogen gas ?
Ans – As per the given 1:8 ratio mass of oxygen gas required to react completely with 1g of hydrogen gas is 8g. Therefore, mass of oxygen gas is required to react completely with 3g of hydrogen
=3 X 8 = 24 g.

Q3 – Which postulate of Dalton’s atomic theory is a result of the law of conservation of mass ?
Ans –The postulate is ‘ Atoms are indivisible particles which cannot be created or destroyed in a chemical reaction’.

Q4 – Which postulate of Dalton’s atomic theory can explain the law of definite proportions ?
Ans – The postulate is ‘ The relative number and kinds of atoms are constant in a given compound’.

PAGE NO – 35

Q1 – Define the atomic mass unit.
Ans – Atomic mass unit is defined as the mass equal to exactly one-twelfth (1/12) of the mass of an atom of carbon-12.

Q2 – Why is it not possible to see an atom with naked eyes ?
Ans – An atom is an extremely small particle whose radius is of the order 10-10 m. And it cannot exist independently.

PAGE No 39

Q1 – Write down the formulae of
(i)  Sodium oxide
(ii)  Aluminium chloride
(iii)  Sodium sulphide
(iv) Magnesium hydroxide
Ans
(i)  Sodium oxide – Na2O
(ii)  Aluminium chloride – AlCl3
(iii) Sodium sulphide – Na2S
(iv) Magnesium hydroxide – Mg(OH)2

Q2 – Write down the names of compounds represented by following formulae :
(i)  Al2(SO4)3
(ii)  CaCl2
(iii)  K2SO4
(iv)  KNO3
(v)  CaCO3
Ans –
(i)  Al2(SO4)   -   Aluminium Sulphate
(ii)  CaCl2     -    Calcium chaloride
(iii)  K2SO   -   Potassium sulphate
(iv)  KNO3    -    Potassium nitrate
(v)  CaCO  -   Calcium carbonate

Q3 – What is meant by the term chemical formula ?
Ans – The chemical formula is the representation of elements present in a compound with the help of symbols and also the number of atoms of each element with symbols. Ex – Water contain two atoms of hydrogen and one atom of oxygen write as H2O.

Q4 – How many atoms are present in a
(i)  H2S molecule and
(ii)  PO43- ion ?
Ans –
(i)  2 atoms of hydrogen + 1 atom of sulphur = 3 atoms
(ii)  1 atom of phosphorus + 4 atoms of oxygen = 5 atoms.

PAGE NO – 40

Q1 – Calculate the molecular mass of
 H2 O2 Cl2 CO2 CH4 C2H6 C2H4 NH3 CH3OH

Ans –
Molecular mass of H = 2 x Atomic mass of H
= 2 x 1 = 2 u
Molecular mass of O2 = 2 x Atomic mass of O
= 2 x 16 = 32 u
Molecular mass of Cl2  = 2 x Atomic mass of Cl
= 2 x 35.5 = 71 u
Molecular mass of CO2 = Atomic mass of C + 2 x Atomic mass of O
= 12 + 2 x 16 = 44 u
Molecular mass of CH= Atomic mass of C + 4 x Atomic mass of H
= 12 + 4 x 1 = 16 u
Molecular mass of C2H= 2 x Atomic mass of C + 6 x Atomic mass of H
= 2 x 12 + 6 x 1 = 30 u
Molecular mass of C2H= 2 x Atomic mass of C + 4 x Atomic mass of H
= 2 x 12 + 4 x 1 = 28 u
Molecular mass of NH3  =  x Atomic mass of N + 3 x Atomic mass of H
= 14 + 3 x 1 =17 u
Molecular mass of CH3OH = Atomic mass of C + 4 x Atomic mass of H + Atomic mass of O
= 12 + 4 x 1 + 16 = 32 u

Q2 – Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of ZN = 65 u, Na = 23u, K = 39 u, C= 12 u, O = 16 u.
Ans  - Formula unit mass of ZnO = Atomic mass of Zn + Atomic mass of O
= 65 + 16 = 81u.
Formula unit mass of Na2O = 2 x Atomic mass of Na +Atomic mass of O
= 2 x 23 + 16 = 62u.
Formula unit mass of K2CO = 2 x Atomic mass of K + Atomic mass of C + 3 x Atomic mass of O
= 2 x 39 + 12 +3 x 16 = 138u.

PAGE NO – 42

Q1 – If one mole of carbon atoms weighs 12 g, what is the mass (in gram) of 1 atom of carbon ?
Ans – One mole of carbon atoms weighs 12 gram
Mass of one mole of carbon atoms = 12 gram
Then, one mole of carbon contains = 12 g = 6.022 x 1023 atoms
So, 1 atom of carbon =       12/6.022 x 1023 gram
= 1.993 x 10-23

Q2 – Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23u, Fe = 56u).
Ans - Moles of Sodium = Mass of sodium / Molar mass of sodium
= 100/23 = 4.34
Moles of iron = Mass of iron / Molar mass of iron
= 100 / 56 = 1.78
THereford,  sodium have more atoms as compare to iron.

EXERCISE QUESTIONS

Q1 – A 0.24 g sample of a compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Claculate the percentage composition of the compound by weight.
Ans –

Q2 – When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.0 g  of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen ? Which law of chemical combinations will govern your answer ?
Ans – If 3 g  of carbon reacts with 8 g of oxygen to produce 11 g of carbon dioxide. When 3 g of carbon is burnt in 50 g of oxygen, then 3 g of carbon will react with 8 g of oxygen. The remaining 42 g of oxygen will be left un-reactive. The answer is governed by the law of constant proportions.

Q3 – What are polyatomic ions ? Give examples ?
Ans – A group of atoms containing positive or negative charge on them are called polyatomic ions. Ex – NH4-- , NO3

Q4 – Write the chemical formula of the following.
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.
Ans –
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Ans –

Q6 – Calculate the molar mass of the following substances.
(a) Ethyne, C2H2
(b) Sulphur molecule, S8
(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31u)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO3
Ans –
(a)  Molar mass of Ethyne, C2H= 2 x 12 + 22 x 1 = 26 g.
(b) Molar mass of Slphur molecule, S8 = 8 x 32 = 256 g.
(c) Molar mass of Phosphorus molecule, P= 4 x 41 = 124 g.
(d) Molar mass of Hydrochloric acid, HCl = 1 + 35.5 = 36.5 g.
(e) Molar mass of Nitric acid, HNO3 = 1 + 14 + 3 x 16 = 63 g.

Q7 – What is the mass of –
(a) 1 mole of nitrogen atoms ?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)
(c) 10 moles of sodium sulphite (Na2So3) ?
Ans –
(a) The mass of 1 mole of nitrogen atoms is 14 g.
(b) The mass of 4 moles of aluminium atoms = 4 x 27 = 108 g.
(c) The mass of 10 moles of sodium sulphite (Na2So3)
= 10 x [ 2 x 23 + 32 +3 x 16 ] = 10 x 126 = 1260 g.

Q8 – Convert into mole.
(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of carbon dioxide.
Ans –
(a) 32 g of oxygen gas = 1 mole
Then, 12 g of oxygen gas = 12 /32 mole = 0.375 mole
(b)18 g of water = 1 mole
Then, 20 g of water = 20/18 = 1.11 moles
(c)44 g of carbon dioxide = 1 mole
Then, 22 g of carbon dioxide =22/44 = 0.5 mole

Q9 – What is the mass of :
(a) 0.2 mole of oxygen atoms ?
(b) 0.5 mole of water molecule ?
Ans  (a) - Mass of one mole of oxygen atoms = 16 g
Then mass of 0.2 mole of oxygen atoms = 0.2 x 16 g = 3.2 g
(b)
Mass of one mole of water molecule = 18 g
Then, mass of 0.5 mole of water molecules = 0.5 x 18 g =9 g

Q10 – Calculate the number of molecules of sulphur (S8) present in 16 g of aluminium oxide.
Ans -

Q11 – Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.
Ans -