The number of moles of hydrogen produced will mirror the number of moles of ZnZn that react, or, from a different perspective, will be half the number of moles of HClHCl that react.
The balanced chemical equation for this reaction is
Zn_((s)) + 2HCl_((aq)) -> ZnCl_(2(aq)) + H_(2(g))Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g)
Now look at the mole ratios you have between ZnZn, HClHCl, and H_2H2: 1 mole of ZnZn needs 2 moles of HClHCl to produce 1 mole of H_2H2.
All you have to do now is figure out exactly how many moles you have. This is done by using the molarity of the HClHCl solution to calculate how many moles of HClHCl you have, since you know that ZnZn is in excess.
C = n/V => n_("HCl") = C * V = "0.40 M" * 400*10^(-3)"L" = "0.160 moles"C=nV⇒nHCl=C⋅V=0.40 M⋅400⋅10−3L=0.160 moles
Therefore, the number of H_2H2 moles will be
"0.160 moles HCl" * ("1 mole H"_2)/("2 moles HCl") = "0.800 moles"0.160 moles HCl⋅1 mole H22 moles HCl=0.800 moles H_2H2
