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

Variables,control,conclusion and finally scientific theory