Determine the mass fraction of calcium in calcium hydroxide.

The mass of calcium hydroxide is 74 grams / mol, the mass of calcium hydroxide can be found by adding the mass of calcium and two hydroxyl groups (40 + 17 * 2 = 74). Knowing the mass of the compound, you can find the mass fraction of each element. The mass of calcium hydroxide is 74 grams and this is 100%, and the mass of calcium is 40 grams and this is X; hence the mass fraction of calcium is equal to: X = 40 * 100/74 = 54.05%.
Mass fraction of oxygen = 32 * 100/74 = 43.24%. Mass fraction of hydrogen = 2 * 100/74 = 2.70%. In general, the compound has one hundred percent, and in the sum, adding the mass fractions of all elements, we are obsessed: 54.05 + 43.24 + 2.7 = 100%.