mass
'no magic to the mole'
amount
molar mass
concentration
solution volume
gas volume
molar gas volume
Avogadro
constant, L
number of
entities, N
Now try the following question.
E3. PRACTICAL ASPECTS OF VOLUME MEASUREMENT IN THE LABORATORY
Measurement of volume is one of the most frequently executed operations in a chemistry laboratory. Although the measurement of volume with such apparatus as pipettes, graduated flasks, syringes, etc. is regarded as a routine procedure, the operation is by no means foolproof.
Preicse volume measurements are required, for example, in:
1. obtaining a known quantity of a sample for analysis;
2. preparation of analytical reagents;
3. preparation of calibration standards;
4. quantitative analysis by titrimetry (aka volumetric analysis).
If the volume measurements are imprecise, the results of an analysis using those measurements will also be uncertain, no matter how skilfully the other steps of the analytical procedure have been carried out.
Measuring the volume of a gaseous product from a reaction in a school laboratory, one suitable for class practical or project work, requires that the gases involved are relatively safe to handle and, ideally, do not require access to a fume hood. Reactions that would produce CO2(g), O2(g), N2(g) or H2(g) from readily available, inorganic or organic starting materials are obvious candidates.
When using a gas syringe to measure gases it is important to keep the syringe free from liquids since gases can dissolve in liquids. It is important that the syringe barrel should move freely within the syringe chamber. Any friction would result in a pressure build-up in the syringe and would lead to an inaccurate measurement, i.e., a lesser amount of gas would be calculated as being present compared to that actually obtained.
N.B. Liquids and solids exhibit practically no change of solubility with changes in pressure: effects are negligible. Gases, as might be expected, increase in solubility with an increase in pressure.