Step-by-step explanation:
Mass number of an atom means the sum of total number of protons and neutrons present in an atom.
On the other hand, atomic number is the total number of protons present in an atom.
Therefore, mass number of phosphorous is 30 g/mol whereas atomic number of phosphorous is 15 g/mol.
Thus, the mass of 1 mole of phosphorus atoms is 30 grams.