How to tackle longer calculation questions for A-level Chemistry
If you’re not confident with calculation questions, the sight of a 6 mark calculation question is likely to send you into a panic. But the truth is, if you approach them in an organised and systematic way, they’re very easy marks.
In this post I’ll teach you a method for tackling the longer calculation questions for A-level Chemistry. Follow this method and even the nastiest looking calculation will start to become a lot clearer. I can’t overstate how much of a difference this method can make – this is one of the first things I teach all my A-level Chemistry students to do and nearly all of them say that this is one of the things that’s made the biggest difference to their confidence with Chemistry. The advice here is aimed mainly at amount of substance/titration type calculations, but the principles apply to all Chemistry calculations.
How to tackle longer calculation questions
Before doing anything on the calculator, follow these 3 steps to help you identify what information you have to work with, how to solve the problem and where to start.
Step 1: Get organised
Write down the equation for the reaction or at least make sure you know the stoichiometry. Assuming all reactants and products are in a 1:1 ratio is a really common mistake.
Then, write down everything you know about each reactant or product underneath it in the equation so that you can clearly see what information you have about each substance. A common mistake is using the wrong volume for the wrong substance, so make sure you’ve matched them up correctly before starting the calculation.
Decide how many significant figures you need for your final answer and
therefore how many significant figures you should keep for each step. If you don’t know the rules for this (no you don’t just round to 3 s.f. every time) then see this post.
Do any unit conversions that you know you’ll need to do at some point but you’re worried that you’ll forget to do later on.
Step 2: Figure out your final step first
If you do nothing else I’ve mentioned in this post, do this. If you can figure out what the final step needs to be gives you a focus for the rest of the calculation. Even if you don’t know how to get to that final step knowing what it needs to be will really help the rest of the calculation fall into place, though it might not be completely clear straight away.
E.g. if you have to calculate the % purity your final step will be : pure mass/total mass x 100
Then, find what piece of information you already have that you can use in the final step. This is really important because you need to make sure you don’t use that piece of information in the wrong place. For example, if you are calculating % purity, you’ll be given the impure mass, which you’ll need to use in the final step. If you don’t realise this is what you have you may end up using the impure mass to calculate moles, which will take your calculation in completely the wrong direction.
Step 3: Find 2 useable pieces of information to get you started
Even if you don’t really know how to do every step of the calculation, you need to find somewhere to start. Usually once you have got started the next steps become clearer. To get started, find 2 pieces of information that fit in the same equation. This will often involve calculating moles (e.g. if you have the concentration and volume of the same substance you can calculate the moles).
If you’re still struggling with the calculations
It may be that you don’t know how to deal with specific problems that come up as part of longer calculations. For example, do you know what to do when you’re given 2 volumes for the same solution? Do you know how to do back titration calculations? If not, it might be specifics such as these that you’ll need to work on. Do as many past paper questions as you can so that you can see as many different past paper questions as possible.
Something else to try is thinking about the practical procedure described in the question – the longer questions are usually presented in the context of a procedure. If you think about what they actually did in the procedure (and why they did it) it usually makes it easier to work out a method for the calculation.