"AKC SCIENCE CLASSES"
CLASS 09 TH (CBSE AND MP BOARD)
CHAPTER 03
ATOMS AND MOLECULES
EXERCISE QUESTIONS AND ANSWERS
Question 1:- A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096g of boron and 0.144g of oxygen. Calculate the percentage composition of the compound by weight.
Ans :-
Mass of the compound = 0.24 g
Mass of boron = 0.096 g
Mass of oxygen = 0.144 g
Mass of boron 0.096 g
Mass of boron
Percentage of boron = ---------------------- x 100
Mass of compound
0.096g
= --------- x 100 = 40%
0.240g
Mass of oxygen
Percentage of oxygen = ----------------------- x 100
Mass of compound
0.144g
= -------- x 100 = 60%
0.240g
Alternative method
Percentage of oxygen = 100 percentage of boron
=100 - 40 = 60%
Question 2:- When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 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 combination will govern your answer?
Ans:-
First we find the proportion of mass of carbon and oxygen in carbon dioxide.
In CO2 , C : O = 12 : 32 or 3 : 8
In other words, we can say that
12.00 g carbon reacts with oxygen = 32.00 g
3.00 g carbon will react with oxygen = 32 x 3
------ = 8g
12
Therefore, 3.00 g of carbon will always react with 8.00 g of oxygen to form CO2 (11g), even if large amount (50.00 g) of oxygen is present. This answer will be governed by ‘the law of constant proportions’.
Question 3:- What are polyatomic ions ? Give examples.
Ans:- The group of atoms which carry a fixed charge (either positive or negative) on them and behave as ions are called polyatomic ions.
Example
(i) Carbonate ion (ii) Sulphate ion
(iii) Ammonium ion (iv) Phosphate ion
Question 4:- Write the chemical formulae of the following.
(i) Magnesium chloride (ii) Calcium oxide
(iii) Copper nitrate (iv) Aluminium chloride
(v) Calcium carbonate
Ans:-
(i) Formula = MgCl2 (Magnesium chloride)
(ii) Formula = CaO (Calcium oxide)
(iii) Formula = Cu(NO3)2 (Copper nitrate)
(iv) Formula = AlCl3 (Aluminium Chloride)
(v) Formula = CaCO3 (Calcium Carbonate)
Question 5:- Give the names of the elements present in the following compounds.
(a) Quick lime (b) Hydrogen bromide
(c) Baking powder (d) Potassium sulphate
Ans:-
(a) Quick lime :- Calcium oxide - CaO
Elements :– Calcium, oxygen.
(b) Hydrogen bromide :- HBr
Elements- Hydrogen, bromine.
(c) Baking powder :- Sodium hydrogen carbonate - NaHCO3
Elements :- Sodium, hydrogen, carbon, oxygen.
(d) Potassium sulphate :- K2SO4
Elements :- Potassium, sulphur, oxygen.
Question 6:- Calculate the molar mass of the following substances.
(a) Ethyne, C2H2
(b) Sulphur molecule, S8
(c) Phosphorus molecule, P4(Atomic mass of phosphorus = 31)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO3
Ans :-
(a) Molar mass of C2H2
= (2 × Atomic mass of C) + (2 × Atomic mass of H)
= (2 × 12) + (2 × 1)
= 26 u
(b) Molar mass of S8
= (8 × Atomic mass of S)
= 8 × 32 = 256 u
(c) Molar mass of P4
= 4 × Atomic mass of P
= 4 × 31 = 124 u
(d) Molar mass of HCl
= Atomic mass of hydrogen + Atomic mass of Cl
= 1 + 35.5 = 36.5 u
(e) Molar mass of HN03
= Atomic mass of H + Atomic mass of N + (3 × Atomic mass of 0)
= 1 + 14 + (3 × 16) = 15 + 48 = 63 u
Question 7:- 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) Molar mass of N atom = Atomic mass of N.
Mass of 1 mol of N atoms = 14 g
(b) Mass of 1 mole Al atoms = 27 g
Mass of 4 moles of Al atoms = 27 × 4 = 108 g
(c) Mass of 1 mole of Na2SO3 = (23 × 2) + 32 + (16 × 3)
= 46 + 32 + 48 = 126 g
Mass of 10 moles of Na2SO3 = 126 × 10 = 1260 g
Question 8:- Convert into mole.
(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of carbon dioxide.
Ans:-
(a) (O2)
Molar mass of oxygen (O2) = 16 x 2 = 32 g
32 g oxygen gas = 1 mol
1 x 12
12 g oxygen gas = --------- mol = 0.375 mol
32g
(b) (H2O)
Molar mass of water (H2O) = 2 + 16 = 18g
18 g water = 1 mol
1 x 20 g
20 g water = ---------- = 1.11 mol
18g
(c) 22g of Carbon Dioxide(CO2)
Molar mass of carbon dioxide (CO2) = 12 + 32 = 44g
44 g CO2 = 1 mol
1 x 22 g
22 g CO2 = -------- mol = 0.5
44g
Question 9:- What is the mass of
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Ans:-
(a) Mass of 1 mole O-atoms = 16 g
Mass of 0.2 mole O-atoms = 16 × 0.2 = 3.2 g
(b) Mass of 1 mole of H2O molecules = 18 g
Mass of 0.5 mole of H2O molecules = 18 × 0.5 = 9.0 g
Question 10:- Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.
Ans:-
Molar mass of sulphur (S8 ) = 32 x 8 = 256 g
Number of S8 molecules in 256 g of solid sulphur = 6.022 x 10²³
Number of S8 molecules in 16 g of solid sulphur
= 6.022 x 10²³ x 16g
---------------
256g
= 3.76 x 10²³ molecules
Question 11:- Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.
[Hint :-The mass of an ion is the same as that of an atom of the same
element. Atomic mass of AI= 27 u.]
Ans:- 1 mole of Al2O3 = 2 ✖ Al + 3 ✖ (O)
= 2 ✖ 27 + 3 ✖ 16
= 54 + 48 = 102 g
1 mole of Al2O3 contains 2 moles of Al³⁺
102 g of Al2O3 contains
= 2 ✖ 6.022 ✖ 10²³ Al³⁺ ions
0.51 g of Al2O3 contains
2 ✖ 6.022 ✖ 10²³
= ----------------------- ✖ 0.051
102
= 6.0 ✖ 10²⁰ Al³⁺ ions
