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Chapter 1 - SOME BASIC CONCEPTS OF CHEMISTRY

1. Calculate the molecular mass of the following:

(i)

(iii)

Answer:

(i)

The molecular mass of water,

• (2
  
  Atomic mass of hydrogen) + (1
  
  Atomic mass of oxygen)
• [2(1.0084) + 1(16.00 u)]
• 2.016 u + 16.00 u
• 18.016
• 18.02 u

(ii)

The molecular mass of carbon dioxide,

• (1
  
  Atomic mass of carbon) + (2
  
  Atomic mass of oxygen)

• [1(12.011 u) + 2 (16.00 u)]

• 12.011 u + 32.00 u

• 44.01 u

(iii)

The molecular mass of methane,

• (1
  
  Atomic mass of carbon) + (4
  
  Atomic mass of hydrogen)

• [1(12.011 u) + 4 (1.008 u)]
• 12.011 u + 4.032 u
• 16.043 u

2. Calculate the mass percent of different elements present in sodium sulphate (

Answer:

The molecular formula of sodium sulphate is

Molar mass of

Mass percent of an element

• Mass percent of sodium:

Mass percent of sulphur:

3. Determine the empirical formula of an oxide of iron which has 69.9% iron and 30.1% dioxygen by mass.

Answer:

% of iron by mass = 69.9 % [Given]

• of oxygen by mass = 30.1 % [Given] Relative moles of iron in iron oxide:

Relative moles of oxygen in iron oxide:

Simplest molar ratio of iron to oxygen:

• 1.25: 1.88

• 1: 1.5

=2: 3

• The empirical formula of the iron oxide is
  
  .

4. Calculate the amount of carbon dioxide that could be produced when

(i) 1 mole of carbon is burnt in air.

(iii) 2 moles of carbon is burnt in 16 g of dioxygen.

Answer:

The balanced reaction of combustion of carbon can be written as:

i. As per the balanced equation, 1 mole of carbon burns in1 mole of dioxygen (air) to produce1 mole of carbon dioxide.

ii. According to the question, only 16 g of dioxygen is available. Hence, it will react with 0.5 mole of carbon to give 22 g of carbon dioxide. Hence, it is a limiting reactant.

iii. According to the question, only 16 g of dioxygen is available. It is a limiting reactant. Thus, 16 g of dioxygen can combine with only 0.5 mole of carbon to give 22 g of carbon dioxide.

5. Calculate the mass of sodium acetate

0.375 molar aqueous solution. Molar mass of sodium acetate is 82.0245 g

Answer:

0.375 M aqueous solution of sodium acetate

∴Number of moles of sodium acetate in 500 mL

Given

Molar mass of sodium acetate = 82.0245 g

Required mass of sodium acetate = (82.0245 g

6. Calculate the concentration of nitric acid in moles per litre in a sample which has a density, 1.41 g

Answer: Mass percent of nitric acid in the sample = 69 % [Given]

Thus, 100 g of nitric acid contains 69 g of nitric acid by mass.

Molar mass of nitric acid (HNO₃ )

• {1 + 14 + 3(16)} g
  

• 1 + 14 + 48

• 63 g
  

• Number of moles in 69 g of HNO₃

Volume of 100g of nitric acid solution

Concentration of nitric acid

∴Concentration of nitric acid = 15.44 mol/L

7. How much copper can be obtained from 100 g of copper sulphate (

• 63.5 + 32.00 + 64.00

• 159.5 g

159.5 g of CuSO₄ contains 63.5 g of copper.

⇒ 100 g of CuSO₄ will contain

of copper.

= 39.81 g

8. Determine the molecular formula of an oxide of iron in which the mass percent of iron and oxygen are 69.9 and 30.1 respectively. Given that the molar mass of the oxide

is 159.69 g

Answer:

Mass percent of iron (Fe) = 69.9% (Given)

Mass percent of oxygen (O) = 30.1% (Given)

Number of moles of iron present in the oxide

= 1.25

Number of moles of oxygen present in the oxide

= 1.88

Ratio of iron to oxygen in the oxide,

2: 3

Empirical formula mass of

Molar mass of = 159.69 g

Molecular formula of a compound is obtained by multiplying the empirical formula with n.

Thus, the empirical formula of the given oxide is

Hence, the molecular formula of the oxide is

9. Calculate the atomic mass (average) of chlorine using the following data:

Answer:

The average atomic mass of chlorine

• 26.4959 + 8.9568

• 35.4527 u

10. In three moles of ethane

i. Number of moles of carbon atoms.

ii. Number of moles of hydrogen atoms.

iii. Number of molecules of ethane.

Answer:

(i) 1 mole of

Number of moles of carbon atoms in 3 moles of

= 2 × 3 = 6

ii. 1 mole of

1 mole of

= 3 × 6.023 × 1023 =

11. What is the concentration of sugar (

if its 20 g

Answer:

Molarity (M) of a solution is given by,

Molar concentration of sugar = 0.02925

12. If the density of methanol is 0.793 kg

Answer:

Molar mass of methanol (

• 32 g
  

• 0.032 kg
  

Molarity of methanol solution

= 24.78

(Since density is mass per unit volume)

Applying,

(Given solution) (Solution to be prepared)

(24.78

• • 0.0252 L

• • 25.22 mL

13. Pressure is determined as force per unit area of the surface. The SI unit of pressure,

Pascal is as shown below: 1Pa =

Answer:

Pressure is defined as force acting per unit area of the surface.

= 1.01332 × 105 kg

We know, 1 N = 1 kg

Then,

1 Pa = 1 kg

14. What is the SI unit of mass? How is it defined?

Answer. The SI unit of mass is kilogram (kg). 1 Kilogram is defined as the mass equal to the mass of the international prototype of kilogram.

15. Match the following prefixes with their multiples:

Answer: