Question: What mass of water, in grams, would have to be decomposed to produce 17.6 L of molecular oxygen at STP?

Work:
n=PV/RT
x=(1 atm)(17.6 L)/(0.0821)(273 k)
x = 1.27348295454546

1.27348295454546 x 2 = 2.54696590909092

Grams= Moles x 18.0
Grams = 2.54696590909092 x 18.0
Grams = 45.8

I think I'm messing up somewhere, but I don't know what I'm doing wrong.

1 answer

Your first mistake (I didn't look past that one) is that you didn't plug in the numbers into the calculator. You have the reciprocal of the number of moles.
n = 1*17.6/[(0.09206)(273)]
n should be 0.7857 and that should be rounded to the correct number of significant figures. (I wonder why you didn't just divide 17.6/22.4 to arrive at the number).