Teacher's notes

This is an interesting experiment and leads to a great discussion afterwards. By doing the experiment students gain a real understanding of quantitative chemistry. They also really enjoy the challenge and you can have fun telling them that no more chemicals (including water) can be used, even to wash up.  Both students and teachers almost invariably approach this problem by using volumetric apparatus. If the school has micro-syringes then these may be chosen but more often than not students opt for 5 cm³ measuring cylinders. These will give an answer but of course the uncertainty is huge. If you look carefully at the two photographs you will see that the most accurate measuring instrument in the laboratory is not being used. The best way to solve the problem is for all the groups to decide to make up one common solution of the acid using a few drops of indicator solution and about 20 cm³ of the water but it is the mass of the solute and all the solvents that should be measured not the volume. Each pair then takes a known mass of about 1 cm³ of the solution and titrates it drop-wise on the balance with the sodium hydroxide solution. This is then repeated. The balance can mass to plus or minus 0.001 g so that the uncertainty when weighing 1 g is still very small (+ or – 0.1%). The only real assumption that has to be made is that the density of the water and the solutions is 1 g cm^-3. By doing this a very accurate answer can be obtained – and there is still 5 cm³ of the water remaining to do the washing up! The answer should be 97.0.

Can you get the students to think of a monoprotic acid with a relative molecular mass of 97?

They usually initially think it must be an organic acid but then realise that no organic acid fits as most with a low molecular mass are liquids and benzoic acid (M_r = 122) which is a solid has a mass far too high. The acid is in fact sulfuric acid in which one of the –OH groups has been replaced by an –NH₂ group to give HSO₃NH₂ which is known as sulfamic acid.  

I once had a student give a brilliant hypothesis for this experiment. He reasoned that as such small quantities are given it is unlikely that any one of the reactants is in a huge excess. If exactly equimolar amounts of the acid and alkali are provided then the relative molecular mass works out to be 96.0. Therefore he hypothesised that the answer would be close to 96.0. I wish I was that clever!