Expert: George Maxwell - 9/15/2007

Question
to measure the amount of gas produced by a chemical reaction what is the most exact method to use a. observation; b. barometric pressure; c. heating and cooling it; d. water displacement

Answer
Hi, and thank you for your question.

The answer is, it depends on the nature of the gas and the work you're doing.

Observation is not an accurate measure of amounts of gas, and can only be used for coloured vapours (e.g. iodine), or a reaction in solution where bubbles are produced.

I'm not sure what you mean by "barometric pressure" (e.g. air pressure). You could measure the amount of gas indirectly by the pressure it produces - e.g. if a flask that holds 1 litre of air normally is pressurised to 2 atomspheres (at constant temperature), it will be holding 2 litres of your gas. This is a bit of a messy and unreliable method though.

Similarly, all that heating and cooling a gas would do would be to change its volume (or change its pressure if the volume is fixed).

Of the methods you listed, therefore, I think water displacement is probably the most accurate. However, its important to realise that this method can also have flaws.
Most common gases dissolve to some extent in water, so as you pass the gas in to the displacement vessel, some of it will be absorbed. The amount of gas absorbed in different experiments will vary irregularly with the temperature of the water, producing inconsistent results.

In most cases, it would be better to use a gas syringe to measure the amount of gas produced. This is a normal syringe that has been sealed to make it gas-tight. You connect a tube from your experiment flask to the nozzle of the syringe, and as the gas enters, it pushes the plunger back along the graduated scale, giving a direct and accurate measure of the volume of gas produced.

Hope this helps, thank you again.