What is the mass fraction of phosphorus in K3PO4?

Given:
K3PO4

To find:
ω (P in K3PO4) -?

Decision:
1) Using the periodic system of D.I. Mendeleev, calculate the relative molecular weight of potassium phosphate:
Mr (K3PO4) = Ar (K) * N (K) + Ar (P) * N (P) + Ar (O) * N (O) = 39 * 3 + 31 * 1 + 16 * 4 = 212;
2) Calculate the mass fraction of phosphorus in potassium phosphate:
ω (P in K3PO4) = N (P in K3PO4) * Ar (P) * 100% / Mr (K3PO4) = 1 * 31 * 100% / 212 = 14.62%.

Answer: The mass fraction of phosphorus in potassium phosphate is 14.62%.