3.32 AS Physical Chemistry - The relative mass of volatile liquids and gases

Volatile means 'vaporises easily'. We can make use of the gas laws to find the relative mass of a volatile liquid by elevating the temperature and finding the volume of a known mass of liquid.

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The gas syringe method

A gas syringe expands to allow the gas inside to reach atmospheric pressure.

gas syringe

Data collection

A typical set of data is shown below:

mass of syringe/g ± 0.005 243.56
mass of syringe + liquid/g ± 0.005 243.71
mass of volatile liquid/g ± 0.01 0.15
temperature oven/K ± 0.5 371.0
atmospheric pressure/kPa ± 10 98.570
gas volume/dm3 ± 0.001 0.085

Data analysis

The ideal gas equation PV = nRT, therefore n = PV/RT

n = (98.57 x 0.085)/(8.314 x 371)

n= 0.00272

Relative mass = actual mass/moles = 0.15/0.00272 = 55.2 (3 sig figs)

Error analysis

Transform actual errors into percentages

    percentage error
mass of volatile liquid/g ± 0.01 0.15 6.67%
temperature oven/K ± 0.5 371.0 0.13%
atmospheric pressure/kPa ± 0.01 98.570 0.01%
gas volume/dm3 ± 0.001 0.085 1.18%
  total 7.99%

The overall percentage error = 7.99%

Therefore the relative mass value obtained through experiment = 55.2 ± 4.4


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