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.
The gas syringe method
A gas syringe expands to allow the gas inside to reach atmospheric pressure.
- 1 The gas syringe is weighed and then approximately 0.1g of the volatile liquid is introduced by means of a syringe through the rubber septum. The syringe and liquid and re-weighed. The mass of liquid is obtained by subtraction.
- 2 The gas syringe is then placed in an oven at approximately 100ºC and allowed to come to equilibrium.
- 3 The volume of the gas inside the syringe is recorded.
- 4 The temperature of the oven is recorded using a thermometer inside the oven.
- 5 The day's air pressure is recorded using a barometer.
- 6 The whole procedure is repeated several times.
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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