Stoichiometry calculation

In summary, stoichiometry is the study of quantitive aspect of chemical reaction.
      When dealing with the stoichiometry calculations, first of all, you have to get a correctly balanced equation. The number in front of each reactants and products means the quantity of each chemicals in the reaction. Then, we can solve the Stoichiometry problems through the ratio of each chemicals.
       
Example :  How many moles of H2 can we get when 20g Iron react with sufficient HCl?
First, we write the eaqution and balance it:
1 Fe(s) + 2 HCl(aq) == 1 FeCl2(aq) + 1 H2(gas)
Second, we change 20g Iron into moles:
the mole mass of iron is 56g/mol.      
20g/(56g/mol)=0.36mol iron
according to the banlanced eqution we made before: 1 mol Fe can make 1 mole H2
So that 0.36 mol/ (1 mol H2/ 1 mol Fe)= 0.36mol H2
We can get 0.36 mol H2 when 20g Iron react with sufficient HCl

Hint: through the balanced equation, we can directly get the ratio of Fe and H2= 1:1.