Answer :
Reaction 2H2 + O2 -> 2 H2O. For the first part of the problem, you wand to know how many molecules of water are produced from 2.02 * 10^23 moecules of oxygen. Then, you use the ratio H2O / O2 = 2 / 1 to determine the number of molecules of H2O are produced from the given amount of oxygen. This is 2.03 * 10^23 molecules of O2 * [2 molecues of H2O / molecule of O2] = 4.06 * 10^23 molecules of water. For the second part, use the ratio 1 moles of O2 / 2 mol of water: 22.5 moles of O2 * 2 mol of H2O / 1 mol of O2 = 45 moles of H2O.
