Determine the theoretical and percent yield of hydrogen gas is 36 g of water undergoes electrolysis to produce hydrogen and oxygen and 3.8 g of hydrogen is collected.

Answer 1

2H2O>>2H2 + O2

so 36 grams of water is 2 moles, so you get 2 moles H2, or 4 grams, theoritical.

Now you got 3.8 grams. Very nice

percent yield: 3.8/4.0 * 100

Answer 2

Here is a worked example showing how to determine theoretical yield and percent yield.
http://www.jiskha.com/science/chemistry/stoichiometry.html