The balanced chemical reaction will be:
2H2O = 2H2 + O2
We are given the amount of water used in the decomposition reaction. This will be our starting point.
17.0 g H2O (1 mol H2O/ 18.02 g H2O) (1 mol O2/2 mol
H2O) ( 32.00 g O2/1mol O2) = 15.09 g O2
Percent yield = actual yield / theoretical yield x 100
Percent yield =10.2 g / 15.09 g x 100
Percent yield = 67.58%