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.

download AQA PSA 05

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


AS Inorganic Chemistry Experiments

Task Possible context
Make up a volumetric solution The preparation of a standard solution of sodium carbonate
Carry out a simple acid-base titration Determine the concentration of unknown hydrochloric acid by titration
Carry out some inorganic tests Tests for anions

AS Physical Chemistry Experiments

Task Possible context
Measure an enthalpy change Use Hess's law to find an unknown enthalpy change, such as the reaction of anhydrous copper(II) sulfate with water to produce hydrated crystals
Determine the Mr of a volatile liquid or the Mr of a gas Determine the Mr of hexane or the Mr of carbon dioxide
Investigate how the rate of a reaction changes with temperature. Investigate the rate of reaction of sodium thiosulfate with acid at different temperatures

AS Organic Chemistry

Task Possible context
Distil a product from a reaction The preparation of ethanal from the oxidation of ethanol or the preparation of cyclohexene from the dehydration of cyclohexanol
Carry out some organic tests Tests for alkene, alcohol, acid, aldehyde
Investigate the combustion of alcohols Use a calorimetric method to measure the enthalpies of combustion in an homologous series of alcohols