Consider the reaction between 52.0 mL of liquid ethanol (C2H5OH; density = 0.789 g/mL) and 25.4 L of O2 at 27.0°C and a pressure of 1.65 atm. The products of the reaction are CO2(g) and H2O(g). Calculate the number of moles of H2O formed if the reaction goes to completion.

Answer 1

This is a limiting reagent (LR) problem; you know that because amounts are given for BOTH reactants.
C2H5OH + 3O2 ==> 2CO2 + 3H2O

grams ethanol = volume x density = approx 40 but that's an estimate as are all of the other calculations here.
mols ethanol = grams/molar mass = approx 0.9.
Use PV = nRT and solve for n for O2 at the conditions listed. My estimate is approx 1.7 mols.

Using the coefficients in the balanced equation, convert mols ethanol to mols H2O. Do the same for mols O2 to mols H2O. It is likely you will obtain two values which means one of them is wrong; the correct value in LR problems is ALWAYS the smaller one and the reagent producing that number is the LR.