Question
Zinc and hydrochloric acid react according to the reaction:
If 0.30 mol Zn are added to hydrochloric acid containing 0.52 mol HCl, how many moles of H2 are produced?
Solution
The reaction is
Stoichiometry shows that
2 mol of HCl need 1 mol Zn
0.52 mol HCl needs 
But moles of Zn acutally present = 0.52 mol.
Therefore, Zn is in excess and HCl is the limiting reagent.
Now 2 mol HCl forms H2 = 1 mol
0.52 mol HCl forms H2 = 
Stoichiometry shows that
2 mol of HCl need 1 mol Zn
0.52 mol HCl needs But moles of Zn acutally present = 0.52 mol.
Therefore, Zn is in excess and HCl is the limiting reagent.
Now 2 mol HCl forms H2 = 1 mol
0.52 mol HCl forms H2 = 
