Questions: What is the percent of phosphorus by mass in phosphoric acid, H3PO4? Use 1.008 g / mol for the molar mass of hydrogen, 16.00 g / mol for the molar mass of oxygen, and 30.97 g / mol for the molar mass of phosphorus. Your answer should be rounded to three significant figures. Do not include units in your answer.

Solution Steps

First, we need to calculate the molar mass of phosphoric acid, \(\mathrm{H}_3\mathrm{PO}_4\).

\[ \text{Molar mass of } \mathrm{H}_3\mathrm{PO}_4 = (3 \times 1.008) + 30.97 + (4 \times 16.00) \]

\[ = 3.024 + 30.97 + 64.00 \]

\[ = 97.994 \, \mathrm{g/mol} \]

Next, we calculate the mass percent of phosphorus in \(\mathrm{H}_3\mathrm{PO}_4\).

\[ \text{Mass percent of P} = \left( \frac{\text{Molar mass of P}}{\text{Molar mass of } \mathrm{H}_3\mathrm{PO}_4} \right) \times 100\% \]

\[ = \left( \frac{30.97}{97.994} \right) \times 100\% \]

\[ = 31.60\% \]

Final Answer