Calculate the mass fraction of phosphorus in the calcium phosphate molecule.

Formula for calculating the mass fraction of an element in a substance:
w = n * Ar (chemical element) / Mr (substance), where n is the number of atoms of a given chemical element.

1) Find the relative molecular weight of the substance (Ca3 (PO4) 2):
Mr (Ca3 (PO4) 2) = 3 * Ar (Ca) + 2 * Ar (P) + 8 * Ar (O) = 3 * 40 + 2 * 31 + 8 * 16 = 120 + 62 + 128 = 310
2) Find the mass fraction of a given element in the substance:
w (P) = 62/310 = 0.2 or 20%

Answer: Mass fraction of phosphorus in calcium phosphate is 20%.