mod 5 and i dont know anything in the previous modules
Are you being tested on content from the previous modules? If not, then don't worry about it rn.
Do you understand the content from mod 5 during class or are u still unsure about parts of it.
If you understand it, the just jump straight into questions from past papers.
If you don't understand small sections of it, try searching up videos to getter a better grasp.
Edit: Another thing I could suggest if i you have a friend that's rlly good a chem, go on a call w/ them and attempt questions together. Pick a question, discuss what your answer would be (or steps of working out) and check your answers. I think it helps solidify your understanding even when your still unsure about the content, especially because you can ask for help easily from your friend without giving too much information when checking the solution.
the weighting for this task is 20%
I get what you mean but it seemingly weighing a lot but don't forget that your school marks together only contribute 50% of your HSC.
So this 20% weighting really is 10% of your HSC mark. It's still something but it probably is the weighing the smallest out of your assessment towards your HSC.