Percent yield of water : 70%
Reaction
2H₂ (g) + O₂ (g) → 2H₂O (g)
mol H₂(MW= 2 g/mol) :
[tex]\tt \dfrac{13.8~g}{2~g/mol}=6.9~mol[/tex]
mol H₂O = mol H₂ = 6.9
mass H₂O(theoretical) :
[tex]\tt 6.9\times 18~g/mol=124.2~g[/tex]
[tex]\tt \%yield=\dfrac{actual}{theoretical}\times 100\%\\\\\%yield=\dfrac{87}{124.2}\times 100\%=70\%[/tex]