Question

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.

Answer 1

First find the total molar mass of H3PO4.

3H + 1P + 4O = 3(1.008) + 30.97 + 4(16.00) = 97.99 g/mol H3PO4

Then divide the molar mass of P in the compound by the total molar mass of the compound. Then multiply by 100% to make it a percent instead of a decimal.

30.97 / 97.99 • 100% = 31.61% phosphorus by mass