% yield = (exp.yield)/(theoretical) x 100%
exp. yield = 2.81g
to find theoretical yield;
1. Construct a balance equation for water electrolysed into H2 and O2. Note the mole ratio between H2O and H2.
2. calculate the mole of H2O using n = m/Mr where m is 80.5g and Mr is molar mass of water.
3. Use the mole in 2 to find the mole of H2 using the mole ratio from the balance equation in 1.
4. Use the mole in 3 to find the mass of H2 using m = nMr where n is the mole in 3 and Mr is the molar mass of H2.
the mass you find will be your theoretical yield.
plug in your values and calculate for % yield.
hope that helps..