Question: how many liters of oxygen are required to produce 2 l of water at stp?
Answer: You'd need 6.55 L of oxygen at STP to form that much water. STP conditions, which are defined as a pressure of 100 kPa and a temperature of 273.15 K, should immediately get you thinking about molar volume of a gas. More specifically, about the fact that 1 mole of any ideal gas occupies exactly 22.7 L at STP.