Using the balanced reaction equation: 2 H2 + O2 ---> 2 H2O then we know that 1 mole of O2 will produce 2 moles of water (H2O).
Next we need to know the molar masses of both H & O (or more specifically those of O2 and H2O, since H2 isn't needed for this calculation. From the Periodic Table of the Elements we know that Hydrogen's (H's) [molar] mass is 1.01 g/mol and Oxygen's is 16.00 g/mol. Therefore O2 has a molar mass of 32.00 [16.00 × 2] and water has a molar mass of 18.02 g/mol [1.01 × 2 + 16.00].
Now we have all of the pieces. I always suggest starting the calculation with what is given in the problem and since there is only one value there we will start with 0.756 of water. Now that 0.756 is missing a unit, so I am going to work it out as if it is grams and then again as if it is moles.
Each semicolon in the following statement separates on ratio in our conversion process. The goal is to turn on thing into another by cancelling out the units. Starting with the mass (grams) of water we first have to get it into moles, then we use the balanced reaction equation to get our stoichiometric [mole] ratio of water to oxygen molecules (the numbers come from the coefficients in front of the reaction equation (1's are always "invisible" [left unwritten]), lastly we round to the correct number of significant figures (since the number from the problem has 3 digits in it (the leading zero doesn't matter) then our answer also needs to have 3 digits in it (the leading 0's don't, matter the trailing one does)).
0.756 g water × 1 mol water/18.02 g water × 1 mol O2/2 mol H2O = 0.0209766926 moles O2 ≈ 0.0210 moles O2
If the units were moles instead, then the process is the same, but we get to skip the first ratio since we only need to cancel out moles of H2O. Starting with the moles of water we use the balanced reaction equation to get our stoichiometric [mole] ratio of water to oxygen molecules (the numbers come from the coefficients in front of the reaction equation (1's are always "invisible" [left unwritten]), lastly we check significant figures (since the number from the problem has 3 digits in it (the leading zero doesn't matter) then our answer also needs to have 3 digits in it (the leading 0 doesn't matter) and see that we have 3 significant figures in our answer too so no rounding is necessary.
0.756 mol water × 1 mol O2/2 mol H2O = 0.378 moles O2
