Finding the percentage of an element in a compound by mass

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

{\%~of~an~element} = \frac{mass~of~an~element~in~the~compound}{relative~formula~mass~M_r} \times 100

Example:

Calculate the percentage by mass of hydrogen in ethanol C2H5OH.

Answer:

Mr of C2H5OH = (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.

{\%~of~hydrogen} = \frac{mass~of~hydrogen}{relative~formula~mass} \times 100 = \frac{6}{46} \times 100 = 13\%

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 × 1023 particles, a number known as the Avogadro Constant.

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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 × 1023 atoms of carbon. The relative formula mass or water is 18. One mole of water weighs 18 grams and contains 6 × 1023 molecules of water.