The chemical reaction would be :
Zn(s)+2HCl(aq) = ZnCl2(aq)+H2(g)
At STP 1 mol of an ideal gas is equa to 22.4 L of the gas. We need to assume that H2 produced is an ideal gas. We can calculate as follows:
42.0 g Zn ( 1 mol / 65.38 g ) ( 1 mol H2 / 1 mol Zn ) ( 22.4 L / 1 mol ) = 14.39 L H2 produced
