Question

please help:

Answer 1

It's probably hydrogen gas.

Answer 2

Notice that the collection chamber is closed only at the top. The collected gas will stay in it only if its less dense than air, so that it naturally floats up instead of sinking. Assuming ideal gas behaviour, the density of a gas is proportional to its molecular weight. Air is mostly N2 (mw 28) and O2 (mw 32). Consider the gases you are given:

(a) CO2, mw 44
(b) Cl2, mw 71
(c) H2, mw 2
(d) HCl, mw 36

Only the H2 is less dense than air.