Given that P-31 is the only natural isotope of phosphorus, what is the mass of one P atom and the mass of Avogadro's number of P atoms, respectively?

The mass of on P atom would be 31 divided by 6.02 * 10^23

The mass of Avogadro's number of P atoms would be 31, since the atomic weight of and element is the mass of exactly one mole of that element, and Avogadro's number is the number of atoms in one mole.