Question

Calculate the molecular mass of a gas if the vapour density of the gas is 22.

Answer 1

Part 1: Vapour density

1. Vapour density is the density of vapour in relation to hydrogen gas.
2. Vapour density is defined as the mass of a certain volume of a substance divided by the mass of the same volume of hydrogen gas.
3. $\mathrm{Vapour}\mathrm{density}=\frac{\mathrm{Mass}\mathrm{of}1\mathrm{molecule}\mathrm{of}\mathrm{gas}\mathrm{or}\mathrm{vapour}\mathrm{at}\mathrm{STP}}{\mathrm{Mass}\mathrm{of}1\mathrm{molecule}\mathrm{of}\mathrm{hydrogen}\mathrm{gas}}$

Part 2: Calculate the molecular mass of a gas

1. The Vapour density of a gas is 22
2. Since a molecule of hydrogen consists of 2 atoms of hydrogen that are H₂ and the mass of one hydrogen atom is 1, therefore,
3. $\mathrm{The}\mathrm{mass}\mathrm{of}\mathrm{the}\mathrm{hydrogen}\mathrm{molecule}\mathrm{is}=2\mathrm{x}\mathrm{the}\mathrm{mass}\mathrm{of}\mathrm{one}\mathrm{hydrogen}\mathrm{atom}.$

$=2\times 1$

$=2\mathrm{g}$

4. $\mathrm{Vapour}\mathrm{density}=\frac{\mathrm{Mass}\mathrm{of}1\mathrm{molecule}\mathrm{of}\mathrm{gas}\mathrm{or}\mathrm{vapour}\mathrm{at}\mathrm{STP}}{\mathrm{Mass}\mathrm{of}1\mathrm{molecule}\mathrm{of}\mathrm{hydrogen}\mathrm{gas}}$

5. 22 = $\frac{\mathrm{Mass}\mathrm{of}1\mathrm{molecule}\mathrm{of}\mathrm{gas}\mathrm{or}\mathrm{vapour}\mathrm{at}\mathrm{STP}}{2}$

6. $22\mathrm{x}2=\mathrm{Mass}\mathrm{of}1\mathrm{molecule}\mathrm{of}\mathrm{gas}$

7. $44\mathrm{g}=\mathrm{Mass}\mathrm{of}1\mathrm{molecule}\mathrm{of}\mathrm{gas}$

Mass of gas whose vapour density is 22 is 44g.