Calculate percent composition of compounds

Before showing you how to calculate percent composition of elements in a compound you need to know: 

1- The molecular mass (sometimes called molecular weight) is the sum of the atomic masses (in amu) in the molecule. We can calculate the mass of a molecule by multiplying the atomic mass of each element by the number of atoms of that element then sum over all the elements.

For example, the molecular mass of H₂O is:

2(atomic mass of H) + atomic mass of O

2(1.008amu) + 16.00amu = 18.02amu

2- The molar mass of a compound (in grams) is numerically equal to its molecular mass (in amu). If the molecular mass of water is 18.02 amu, so its molar mass is 18.02 g.

The number of molecules in one mole of water weighs 18.02 g is equal to 6.022x10²³ H₂O molecules.

3- Formula mass is used to express mass of ionic compounds such as NaCl and MgO. For example, the formula mass of NaCl is sum of the atomic masses of Na⁺ and Cl^- ions.

Thus, the formula mass of NaCl is the mass of one formula unit:

Formula mass of NaCl= 22.99amu + 35.45amu

= 58.44amu

Percent composition of compounds:

The percent composition by mass is the percent by mass of each element in a compound. If we divide molar mass of one mole of an element by the molar mass of the compound we get the percent composition of the element. This means,

You have to remember that you need to multiply by the number of moles (n).

Example, to find out the percent composition of hydrogen in water we need to calculate the molar mass of water;

Molar mass of water = 2(1.008g) + 16.00g = 18.02g

So the percent composition of hydrogen in water is equal;

You could see that the value of "n" in the above equation is equal to 2 because there is 2 hydrogen moles in each one mole of water.

And the percent composition of oxygen is equal;