What mass of water, in grams, would have to be decomposed to produce 26.3 L of molecular oxygen at STP?

Answer 1

Since 22.4 liters of a gas is 1 mole at STP, you would need 26.3/22.4 = 1.17 moles of O2.

I assume you are talking about the electrolysis reaction

2 H2O -> 2H2 + O2

Clearly, you need twice as many moles of water, or 2.34 moles, which would have a mass of 42.1 g.