I was working 1:1 with a student a few weeks ago on solubility product calculations.
I always like to start with running through the background theory before we jump into the maths, so I started with an equation to show a sparingly soluble salt in equilibrium with its ions in solution …
BaSO4(s) ⇌ Ba2+(aq) + SO42-(aq)
The equilibrium constant for this system is written in exactly the same way as any other – the concentrations of the products at equilibrium divided by the concentrations of the reactants.
and we find that Ksp = 1.10 x 10-10 mol2 dm-6 for this system.
Question …. in terms of the CHEMISTRY what does that number mean?
Well, it’s a really tiny number!
Which means the combined concentrations of the products must be very very very small compared with the concentration of the reactant (we have clearly divided a very small number by a very large number).
In terms of the chemistry, it means that the position of equilibrium lies very very very strongly to the left and at equilibrium hardly any of the barium sulfate salt has actually dissolved.
This makes sense, after all barium sulfate is a sparingly soluble (pretty insoluble) salt.
Key takeaways:
- Being able to do all the types of calculation in Chemistry A level is only part of the picture
- You have to understand what your final answer is telling you about the Chemistry!
This is where so many students fall down in the exam. And to be honest, students who don’t truly understand the chemistry find the calculations themselves really difficult, especially in unfamiliar contexts under exam pressure.
Action points:
- When you are writing example calculations into your notes, annotate your final answer with a statement explaining what that number actually means in a different colour (or add a post-it note)
For example, if you’ve calculated the acid : salt ratio for a buffer, you now know in what proportions you should make up that buffer.
Or maybe you’ve calculated the value of T in ΔG = ΔH – TΔSsys, you now know the minimum temperature at which the reaction becomes feasible.
If this has been useful then please forward it on
and thank you very much for reading,
