Does that mean you'll get 100%? And what papers are you doing?
Do papers from every different school as well as CSSA, Independent, HSC papers.
@OP if you don't know the material well yet (and that's completely fine), do the papers open book and then mark your answers and compare them to the marking criteria. Eventually you will see that some questions are repeated like 8374382 times and you'll be able to do them with your eyes closed without thinking too much. So keep practising until it's perfect. And chem is hard, so your past paper practice will not go to waste. Doing them open book initially allows you to get comfortable with structuring your answers, once you're good with that, you'll remember your information better.
It's like if you keep reading your notes, you wont remember them. If you actually do something with them (ie use them to answer past paper questions) then it's a better form of revision and you're doing two things at once, hence it's way more efficient.