Mandatory Lab Ideas, 1.3, determining the molar mass of a gas by using the ideal gas equation.

These posts are developing into a bit of a theme. So far, in July we looked at mandatory lab 1.2, ‘the obtaining and use of experimental data for deriving empirical formulas from reactions involving mass changes’, while last month is was 1.3, ‘use of the experimental method of titration to calculate the concentration of a solution by reference to a standard solution’.

This month, I thought I would look at 1.3 again but this time the mandatory lab / skill that involves ‘Obtaining and the use of experimental values to calculate the molar mass of a gas from the ideal gas equation.’

Let us remind ourselves of the ideal gas equation:

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Where n = number of moles. Given that moles also equals:

Moles = mass / Mr

We can substitute this into the equation to give us:

pV = [mass x R x T] / Mr

And rearranging to make Mr the subject:

Mr = [mass x R x T] / [p x V]

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As with previous posts, it goes without saying that you, the teacher are fully responsible for ensuring these labs are carries out safely and it is imperative that you carry out the appropriate risk assessments before engaging in any of the labs listed.

In my experience, there are three types of labs you can do to satisfy this mandatory lab:

1, The simplest method that gives, surprisingly, the best results (Occam’s razor in action?) is to do a quick, back of the envelope calculation and use some 1.0M HCl and Mg ribbon cut into approx. 1cm, strips (you’ll need to know the mass of one Mg strip) – you need to produce approx. 100mL of H2 gas and collect this in a gas syringe. As I said, it gives remarkably good answers.

2, Put a known mass of volatile liquid into a sealed gas syringe using a hypodermic syringe (be careful!) and use a boiling beaker of water to vaporize the gas. It sounds dangerous but works fine, although the results are not great.

3, Use the same method but instead of putting the gas syringe into a beaker of boiling water, heat it using an electric heater designed for this purpose – involves investing in apparatus that may only get used once or twice a year.

As ever, it would be really nice if you could share any other labs you may have carried out with your classes that are not listed here. I would love to read about them.

Next month, I intend looking at mandatory lab 5.1, ‘a calorimetry experiment for an enthalpy of reaction’.

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