steps...
1) convert mass H2O to moles H2O
2) use coefficients of balanced equation to convert to moles O2
3) convert moles O2 to L O2.
solution...
10.5 g H2O x (1 mole H2O / 18.0 g H2O) = 0.583 moles H2O
from the balanced equation, 1 mole O2 --> 2 moles H2O.. therefore...
0.583 moles H2O x (1 mole O2 / 2 moles H2O) = 0.292 moles O2
from here you can either use the specific form of the ideal gas law for STP...
0.292 moles O2 x (22.4 L O2 / mole O2) = 6.54 L O2
or you can use the more general form of the ideal gas law...
V = nRT/P = (0.292 moles) x (0.0821 Latm/moleK) x (273 K) / (1 atm)
V = 6.54 L O2
Hope this helps!
