Problems Based on Mole Concept

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Example: 5

How many moles of sodium hydroxide are present in 3200 g of it?

(Na=23, O=16, H=1)

Solution:

Gram molecular mass of NaOH = (23 + 16 + 1) = 40 g

Mass of 1 mole of NaOH = 40g

No. of moles present in 3200 g of NaOH = 80 moles.

Example: 6

Calculate the number of moles of carbon dioxide present in 1.1 g of carbon dioxide. (C=12, O=16).

Solution:

Gram molecular mass of carbon dioxide = {12+ (2 x 16)} = 44 g

Mass of 1 mole of CO₂ = 44 g

No. of moles of CO₂ present in 1.1 g of CO₂ = ?

Mole : Mass

1 : 44 g

x : 1.1

Hence the number of moles present in 1.1 g of CO₂ = 0.025 moles.

Example: 7

How many grams of pure sulphuric acid is present in 0.05 mole of it?

(H=1, S=32, O=16)

Solution:

Gram molecular mass of H₂SO₄ = (2 x 1) + (1 x 32) + (4 x 16) = 98 g

Mass of 1 mole of H₂SO₄ = 98 g

No. of grams of H₂SO₄ present in 0.05 mole of it =?

Mole : Mass

1 : 98

0.05 : x

x = 0.05 x 98 = 4.9 g

Mass of H₂SO₄ present in 0.05 mol of H₂SO₄ = 4.9 g

Example: 8

Calculate the number of moles of hydrous copper sulphate present in 1 kg of it.

(Cu=64, S=32, O=16, H=1)

Solution:

Formula of hydrous copper sulphate = CuSO₄.5H₂O

Gram Molecular Mass of CuSO₄.5H₂O

= (1 x 64) + (1 x 32) + (4 x 16) + 5 {(1 x 2)+ 16}

= 64 + 32 + 64 + 90 = 250 g

Mass of 1 mole of CuSO₄.5H₂O = 250 g

No. of moles of CuSO₄.5H₂O present in 1 kg or 1000 g of it is

Moles : Mass

1 : 250

x : 1000

x = 4 moles

Number of moles of CuSO₄.5H₂O present in 1 kg of it = 4 moles.