Calculating Molecular Weight using Vapour Density

Vapour density is the ratio of the mass of a gas to the mass of an equal volume of hydrogen at the same temperature and pressure. It is an important parameter for calculating the molecular weight of a substance. Here's how:

1. Collect a sample of the substance in its gaseous state.
2. Weigh an empty vessel and record its mass.
3. Fill the vessel with the gas and weigh it again.
4. Subtract the mass of the empty vessel from the mass of the vessel with gas to get the mass of the gas.
5. Measure the temperature and pressure of the gas.
6. Determine the volume of the vessel using a gas volume calculator.
7. Calculate the number of moles of gas using the ideal gas law: PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the gas constant, and T is the temperature in Kelvin.
8. Divide the mass of the gas by the number of moles to get the molecular weight of the substance.

Example:

Suppose we have a gas that weighs 2.5 grams and occupies a volume of 1.5 liters at a temperature of 25°C and a pressure of 1 atm. The vessel weighs 1 gram when empty.

1. Mass of gas = (2.5 g + 1 g) - (1 g) = 2.5 g
2. Temperature = 25 + 273 = 298 K
3. Pressure = 1 atm
4. Volume = 1.5 L
5. Calculate the number of moles of gas using the ideal gas law: PV = nRT
n = PV/RT = (1 atm) x (1.5 L) / [(0.082 L atm/mol K) x (298 K)] = 0.058 mol
6. Divide the mass of the gas by the number of moles to get the molecular weight:
Molecular weight = 2.5 g / 0.058 mol = 43.1 g/mol.

Therefore, the molecular weight of the gas is 43.1 g/mol.