Question

5.6 litres of gas Z weighs 32 g at STP. What is the molecular weight and vapour density of Z?

Hint:

Vapour density is a ratio and has no units, whereas molecular weight is usually expressed in atomic mass units (amu) or grams per mole!

Answer 1

Step 1: Calculate Molecular Weight:
According to the molar volume concept, 22.4 litres of any gas at STP is equal to its molecular weight in grams.

Weight of 5.6 L of gas Z = 32 g
Weight of 1 L of gas Z = 32 / 5.6
Weight of 22.4 L of gas Z = (32 / 5.6) × 22.4 = 128 g
Therefore, the Molecular Weight of Z is 128.

Step 2: Calculate Vapour Density:
The relationship between molecular weight and vapour density is given by the formula:
Molecular Weight = 2 × Vapour Density
Vapour Density = Molecular Weight / 2 = 128 / 2 = 64