Question

How do you calculate the percent composition of water?

Answer 1

Percent composition of water:

• The percentage composition of any given compound, which is given by;

$\mathrm{Mass}\%\mathrm{of}\mathrm{Element}=\frac{\mathrm{Mass}\mathrm{of}\mathrm{Element}}{\mathrm{Mass}\mathrm{of}\mathrm{Compound}}\times 100$

• We know that the molecular weight of water is about 18 g/mol and the mass of oxygen is about 16 g/mol, also the mass of hydrogen is about 2 g/mol.
• The percentage of oxygen in water can be calculated as follows

$\text{Mass of Element}={\displaystyle \frac{\text{Mass of Element}}{\text{Mass of Compound}}}\times 100$
$\begin{array}{rl}& ={\displaystyle \frac{16}{18}}\times 100\\ & =88.88\mathrm{\%}\end{array}$

• We can now calculate the percentage of hydrogen in water as follows:

$\begin{array}{rl}& \text{Mass of Element}={\displaystyle \frac{\text{Mass of Element}}{\text{Mass of Compound}}}\times 100\\ & ={\displaystyle \frac{2}{18}}\times 100\\ & =11.11\mathrm{\%}\end{array}$

Therefore, in this way, we can calculate the percent composition of water, which is $11.11\mathrm{\%}$ by mass of hydrogen and $88.88\mathrm{\%}$ by mass of oxygen.