How many cations are there in 20.0 g of sodium phosphate?

Sodium phosphate has a chemical symbol of Na₃PO₄. When this dissociates into its ionic components then, these are Na⁺ and PO₄³⁻. The first thing that needs to be done is to divide the given mass of the substance by its molar mass to reveal how many moles of substance is present as shown below.

 molar mass of Na₃PO₄ is 163.94 g/mol

        number of moles = (20 g)(1 mol / 163.94 g) = 0.122 moles

Then, multiply the number of moles by the Avogadro's number to reveal how many formula units are present.

         number of formula units = (0.122 moles)(6.022 x 10²³ formula units/mol)
       number of formula units = 7.35 x 10²²

Since, in every formula unit there are 3 Na⁺ then, the answer should be equal to,
        number of Na⁺ ions = (3)(7.35 x 10²²) = 2.2 x 10²³ ions

2.2 x 10²³ ions