Well, the best way to know the products will come down to experience. However, lets at least start to understand possible products by actually writing out the possible double replacement reactions, and lets see where we can go from there. Also, did you notice any properties of the gas released as bubbles? Any interesting (or not) smell?
I was working in a group. The thing is, my partners were the ones that did that part of the experiment. They only saw bubbles and gas, but since it involves chlorine, I'd assume they'd have felt that smell because it'd be strong.
This is how it'd look like...
HCl + H2SO4--> *Impossible, you'd end up with the same thing*
HCl + Na2CO3--> H2CO3 + NaCl but none are gases.
I really need help.
No properties seen (just the gas).