The way you set up equations for solving is very strange. Maybe it's some new way that schools are teaching but I'm unfamiliar with it.

Another suggestion for you. You should actually learn the science and principles behind the equations and heuristics the teachers give you. If you understood what the equations actually meant you wouldn't be so confused as to what values to plug in where. You wouldn't even think of problems as things to plug values into.

Back to it. You've told us that you know 1 mole of anything (at STP) will occupy 22.4dm

^{3} of volume. And you have 3.25dm

^{3} of H

_{2}.

I asked the questions that I did because that's the order in which you should be thinking through this problem. It's a simple problem but it can be broken down into even smaller steps. So...

- How many moles of H
_{2} do you have if you have 3.25dm^{3} at STP? (Hint, use your 22.4dm^{3}/mol constant) - How many moles of Zn did you need to make that many moles of H
_{2}? (Hint, use your balanced chemical equation Zn + 2 HCl ZnCl_{2} + H_{2} - What's the mass of that many moles of Zn? (Hint, the molar mass of Zn is 65.38g/mol).

I don't think it's possible for me to help you any more without actually telling you the answer. Please, please consider going back into your notes to find examples from your teacher. And you should probably reach out to your teacher for more assistance. He or she will hopefully be able to solidify some of these concepts for you.