Mandatory Lab Ideas, 1.2, Empirical Formula

As you well know, the IB now tells us that we must carry out up to twelve mandatory labs or skills (I say up to as there is some debate whether 9.1, the application of the Winkler method is mandatory or not and the ‘single replacement labs’ occur twice, in 15.1 and 19.1).

So I thought I would focus the next few blog posts on some of these mandatory labs, starting with the first on one the list, namely, topic 1.2, ‘the obtaining and use of experimental data for deriving empirical formulas from reactions involving mass changes.’

This is a good lab to start with as there are so many different ways of carrying it out.

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.

The first, and possibly most common method doesn’t actually give great results and there are a myriad of other labs to try out.

The first lab that springs to mind is with the formation of MgO by heating Mg in a crucible with a lid on. The Mg needs to be heated so that the crucible begins to glow and the lid needs to be taken off quickly enough to let more air inside the crucible whilst not letting any MgO smoke / vapour escape. The Mg is heated until it stops glowing when the lid is removed. If the mass of Mg is taken and then the mass of MgO taken, the mass of O can be found and hence the empirical formula. Don’t get your hopes up though, it usually comes out as being as something like Mg2O! That said, you can have a good discussion about why this happens and why the formula wouldn’t be something like MgO2 (ie, why is there less oxygen than expected as opposed to more).

Three other alternatives to consider – used hydrated copper sulfate and heat it to make it anhydrous. This gives surprisingly good results. Why does the anhydrous copper sulfate need to be weighed when it is still hot?

The formation of CuO by heating Cu turnings in a tube joined connected by two gas syringes. One syringe is full and the other empty, air can then be passed over the Cu as it is being heated. Don’t push too hard as you will blow the Cu into one of the gas syringes!

A final lab involves getting a boiling tube a making a hole in the end around the size of a large needle. CuO is put into the boiling tube and gas from a gas tap is passed over it. The escaping gas goes through the small hole and can be ignited (Don’t have the gas coming out too quickly or the flame will not light or worse still not be anchored to the boiling tube). The CuO can now be heated with a bunsen. As gas contain a lot of hydrogen (much more than you realise!) the hydrogen will reduce the CuO and produce water. Once all the CuO has been reduced into copper, the mass of Cu can be obtained if the original mass of CuO was known. This can then allow the empirical formula to be calculated (Why do you need to take the mass of the reduced Cu while it is still hot?)

I hope these labs give you some ideas, as ever, it would be great to hear from you either about your experience of using these labs or with any alternative methods you may also have