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WAEC Chemistry 2014 Paper

Question : 17

Total: 45

Solution:

Concept:

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

22.4\ \mathrm{dm}^3

Molar mass is the mass of one mole of the gas.

Explanation:

First convert the volume of gas Q from

\mathrm{cm}^3

\mathrm{dm}^3

Since

1000\ \mathrm{cm}^3 = 1\ \mathrm{dm}^3

, we get:

800\ \mathrm{cm}^3 = 0.800\ \mathrm{dm}^3

Now calculate the number of moles of gas Q using the molar volume.

\text{Amount of gas} = \dfrac{\text{Volume}}{\text{Molar volume}}

\text{Amount of gas} = \dfrac{0.800\ \mathrm{dm}^3}{22.4\ \mathrm{dm}^3\ \mathrm{mol}^{-1}}

\text{Amount of gas} = 0.0357\ \mathrm{mol}

Next, use the relation between mass, amount, and molar mass.

\text{Molar mass} = \dfrac{\text{Mass}}{\text{Amount}}

\text{Molar mass} = \dfrac{1.0\ \mathrm{g}}{0.0357\ \mathrm{mol}}

\text{Molar mass} = 28.0\ \mathrm{g\ mol}^{-1}

Answer:

The molar mass of gas Q is

28.0\ \mathrm{g\ mol}^{-1}

, so the correct option is C.

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