The balanced chemical reaction is:
N2 + 3H2 = 2NH3
We are given the amount of H2 being reacted. This will be our starting point.
26.3 g H2 (1 mol H2 / 2.02 g H2) 2 mol O2/3 mol H2) ( 17.04 g NH3 / 1mol NH3) = 147.90 g O2
Percent yield = actual yield / theoretical yield x 100
Percent yield = 79.0 g / 147.90 g x 100
Percent yield = 53.4%