what is the percentage of oxygen by mass in a 200gram sample of CuSo4 * 5H2O? I just added all the masses, got the mass of oxygens. mass of oxygen / total mass * 100 % = percentage of oxygen in CuSo4 * 5H2O. What I don't understand is why does it say 200 gram sample? mass of oxygen = 144 total mass = 250 is it because no matter what the total mass is, the percentage of oxygen doesn't change becuz the mass of oxygen will also inc or dec at the same rate as the total mass?