The coefficients in a reaction become the ratio values in the denominator of the ratios between moles of material in a reaction.  For example:

for:    2H₂  + O₂ -->  2H₂O

This leads to the following ratio for compounds reacting and being produced:

 
 
n stands for number of moles (a similar equation could be written for number of molecules).

For whatever problem utilizing this reaction select the relavent two ratios and solve using moles.

In a reaction, one must always use a relationship such as the one above.  I.E. moles to moles (or numbers to numbers).