In the reaction Mg (s) + 2HCl (aq) H2 (g) + MgCl2 (aq), how many moles of hydrogen gas will e produced from 75.0 milliliters of a 1.0 M HCl in an excess of Mg?

This question uses mole ratios. firstly, we look at how many moles of H2 are produced for every mole of HCl: from 1 Hcl, we get half a mole of H2. therefore, however many moles of HCl are present in the 0.075 L of 1.0M HCl, half that number of H2 moles will be produced.

n=cv

=0.0750 x 1.0

=0.075 mol HCl

n (H2) = 0.5 x n (HCl)

=0.0375 moles of H2