What mass of water, in grams, would have to be decomposed to produce 25.1 L of molecular oxygen at STP?
how many moles of O2 is 25.1 liters (STP):answer: 25.1/22.4
2H2O>>2H2 + O2
so according to the balanced equation, you need 2*25.1/22.1 moles of water, convert that to grams.