Chemists use relative atomic masses and relative formula masses to carry out mole calculations.

To find what percentage of a compound is a particular element, you must first calculate the relative formula mass (M_{r}) of the compound. Then, use the following equation:

Example:

Calculate the percentage by mass of hydrogen in ethanol C_{2}H_{5}OH.

Answer:

**M _{r} of C_{2}H_{5}OH = (2 × 12) + (5 × 1) + (16 + 1) = 46**

There are six atoms of hydrogen in the compound, so the mass of hydrogen is 6 × 1 = 6.

Atoms and molecules are too small to count individually. Instead, chemists use a quantity called **amount of substance**, measured in a unit called the mole (mol).

A mole contains 6 × 10^{23} particles, a number known as ** the Avogadro Constant.**

The mass of one mole of any substance is numerically equal to its relative formula mass or relative atomic mass

For example, one mole of carbon is 12 g and contains 6 × 10^{23} atoms of carbon. The relative formula mass or water is 18. One mole of water weighs 18 grams and contains 6 × 10^{23} molecules of water.