Relative molecular mass of a volatile liquid

Worked example

A sample of a volatile liquid with mass 0.518 g was injected into a gas syringe and vaporised. The vapour occupied 122.1 cm³ at 372 K and 1.01 × 10⁵ Pa. Calculate the relative molecular mass of the liquid.

1. 1. Ideal gas equation: solve for n(vapour)

   $n = \dfrac{pV}{RT}$

   = 0.003987 mol

2. 2. Moles from mass: solve for M

   $M = \dfrac{m}{n}$

   = 129.9 g mol⁻¹

Answer: M = 130 g mol⁻¹

Method

n = pV/RT, with p in Pa, V in m³ and T in K. Then M = m/n; Mr is the same number without units.

Formulas used

• Ideal gas equation

  $pV = nRT$

• Moles from mass

  $n = \dfrac{m}{M}$

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