V= 179,2 L H₂
n = [tex] \frac{V}{Vm} [/tex] = 179,2/22,4 = 8 moli H₂
m H₂ = 8*2 = 16g
n = m/M = 122,4/18 = 6,8 moli H₂O
H₂ + 1/2O₂ ⇒ H₂O
2g H₂..................1 mol H₂O
xg H₂..................6,8 moli H₂O
x = 13,6g H₂
η = [tex] \frac{mp}{mt} [/tex]*100 = [tex] \frac{13,6}{16} [/tex]*100 = 85 %