Percent composition is the percent by mass of each element present in a compound.

Water, H₂O, is the first example.

One mole of water is 18.0152 grams.

In that compound, there are two moles of H atoms and 2 x 1.008 = 2.016 grams. That's how many grams of hydrogen are present in one mole of water.

There is also one mole of oxygen atoms weighing 16.00 grams in the mole of water.

To get the percentage of hydrogen, divide the 2.016 by 18.015 and multiply by 100, giving 11.19%.

For oxygen it is 16.00 ÷ 18.015 = 88.81%.

Notice that you can also minus hydrogen's percentage from 100%. You can always do this with the last part of this type problem.

Glucose, C₆H₁₂O₆, is the second example.

1. figure out the molar mass from the formula.
2. figure out the grams each atom contributes by multiplying the atomic weight by the subscript.
3. divide the answer for each atom by the molar mass and multiply by 100 to get a percentage.

Remember, you may figure out the last percentage by subtracting the total percent from 100, as will be done in a moment.

Step One: mass of one mole = 180.16 g

Step Two:

Carbon = 6 x 12.011 g = 72.066 g
Hydrogen = 12 x 1.008 = 12.096 g
The oxygen percentage will be arrived at by subtraction.

Step Three:

Carbon's percentage: (72.066 g / 180.16 g) x 100 = 40.00 %
Hydrogen's percentage: (12.096 g / 180.16 g) x 100 = 6.71 %
Oxygen's percentage: 100 - (40.00 + 6.71) = 53.29 %

You may calculate the oxygen percentage if you wish.