Question

(a) Explain Avogadro's law. [1 Mark]
(b) When magnesium metal is added to water, hydrogen gas, ${\text{H}}_2$ is released. If 72 g of hydrogen gas is liberated in this reaction, how many number of moles of hydrogen gas is produced?
$\text{Mg}$ (s) + $\text{2}$${\text{H}}_2\text{O}$ (l) $\to$ $\text{Mg}{\text{(OH)}}_2$ (aq) + ${\text{H}}_2$ (g) ​ [2 Marks]

c)How the volume of a gas is related to the number of moles? What is the volume of one mole of a gas at STP? [1 Mark]

Answer 1

a) Amedeo Avogadro put forward a hypothesis based on the relation between the number of molecules and the volume of gases.
Avogadro’s law states that equal volumes of all gases under the same conditions of temperature and pressure contain an equal number of molecules. [1 Mark]

b)Given mass of hydrogen (${\text{H}}_2)$ = 72 g
Gram molecular mass of ${\text{H}}_2$ = 2 g

Number of moles = $\frac{\text{Given mass}}{\text{Gram molecular mass}}$ [1 Mark]
$\therefore$ Number of moles of ${\text{H}}_2$ = $\frac{72}{\text{2}}$ = 32 mol
[2 Mark]

c) At constant temperature and pressure, the volume of a gas depends on the number of molecules, and not on the size and type of molecules. Therefore, as the number of moles of a gas increases, volume of the gas increases. [0.5 Mark]
One mole of any gas at STP occupies 22.4 L of volume. [0.5 Mark]