How do you find the atomic mass number of phosphate?

Hint: Every atom is composed of a nucleus and this nucleus is made up of protons and neutrons where protons is positively charged and neutrons are neutral in charge and electrons which are negatively charged are revolving around the nucleus.

Complete answer:
Atomic number and mass number are always whole numbers because these are calculated by counting whole objects i.e. protons, neutrons, and electrons. The sum of the mass number and the atomic number for an atom corresponds to the total number of subatomic particles present in the atom. The mass number reports the mass of the atoms present in the nucleus in atomic mass units represented by the term amu.
We know that the modern periodic table is arranged in increasing atomic number and mass number. Phosphate is the molecule represented by the chemical formula $P{{O}_{4}}^{3-}$ in which one atom of phosphorus and 4 atoms of oxygen are present. Now to find the atomic mass unit of phosphate we have to add atomic masses of all the atoms present in it.
Here atomic mass of oxygen is 15.99 here 4 atoms of oxygen are present and atomic mass of phosphorus is 30.97, atomic mass of phosphate will be:
$4\times 15.99+1\times 30.97=94.93$amu.

Note:
Atomic weight is measured in the term of isotopes where isotopes are those compounds which have the same chemical element but different atomic mass numbers. Atomic mass number is given by total number of protons and neutrons.