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JAMB Chemistry 2024 Paper

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Question : 134

Total: 138

What is the vapour density of 560cm

^{3}

of a gas that weighs 0.4g at s.t.p?

6.0
8.0
16.0
32.0

Solution:

Concept:

Vapour density is half the molar mass of a gas.

Explanation:

At s.t.p., one mole of any gas occupies

22400 \, \text{cm}^3

.

Calculate the number of moles of the gas:

n = \frac{V}{\text{molar volume}} = \frac{560}{22400} = 0.025 \, \text{mol}

Use the mass and moles to find the molar mass:

M = \frac{m}{n} = \frac{0.4}{0.025} = 16.0 \, \text{g mol}^{-1}

Vapour density is related to molar mass by:

\text{Molar mass} = 2 \times \text{vapour density}

Therefore:

\text{Vapour density} = \frac{16.0}{2} = 8.0

Answer:

B.

8.0

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