Sorry this reply is too late for you, but hopefully it will still help for the exam
Firstly, if you are having problems with all the sorts of questions involving moles, you should approach your teacher for help - I'm sure they'd be happy to go through it with you.
I'd also recommend the notes on this site HSN11200 - Topic 2 - Enthalpy Changes/The Mole
is probably the most relevant, but you might find the other Higher Chemistry
resources useful too.
In the file I've linked to above, you'll find lots of examples of mole-based calculations. It's all summarised in a diagram on the last page - basically, every question is asking you to use those relationships to get the answer.
The method used in the notes is proportion, which requires you to think a bit. I was taught to do the questions that way, and it works for me
Other teachers use formulae for that sort of thing, and I'm sure there are other members here who can help you with those if you want.
I've not done chemistry in a while, so I might be wrong, but these are the steps I think you need to go through in this question:
- Find the number of moles of sodium thiosulphate in the second equation. You must have it's concentration, because I can't think how you would do it otherwise
- The number of moles I2 reacting with the thiosulphate is 1/2 the number of moles thiosulphate
- This number of moles I2 must have been produced in the first equation, and the number of moles O3 is therefore the same
- Using the molar volume given, you can find the volume of ozone
I hope that's helpful