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Percentage mass of an element in a compound

The percentage mass of an element in a compound is the proportion of mass that is contributed by a particular element.

Using a compound $$\ce{XYZ}$$ as an example, the percentage mass of $$\ce{X}$$ can be calculated using the following formula:

$$$\text{Percentage mass of X} = \displaystyle{\frac{\text{A}{_r}\text{ of X}}{\text{M}{_r}\text{ of XYZ}}} \times 100\%$$$

Water is approximately 11% hydrogen by mass based on the calculation: $$\displaystyle{\frac{1+1}{1+1+16}} \times 100\%=11\%$$. The other 89% by mass is oxygen.

The percentage mass could be used to determine the mass of the products if the compound were to be decomposed.

Only about $$3.5 \times 10^{-3}\%$$ of pure gold (shown on the red paper) can be extracted from the gold ore shown in this picture.
Only about $$3.5 \times 10^{-3}\%$$ of pure gold (shown on the red paper) can be extracted from the gold ore shown in this picture.