If The mass of M in M2O is 63% then ONE M is 31.5% and the mass of O is 37% Percentage is by mass. You can use the percent as mass for the second part of the proglem. Assume the compound M2O has a mass of 100 g. Then M = 31.5 g, and O=37g. The mass of the compound MO would be: 31.5 g + 37g = 68.5 ...