Question

If a sample of K3PO4 is found to contain 1.81 grams of K+ ions, how many PO3- 4 ions are present?

Answer 1

none. There are no PO3-4 ions present. If you mean PO4^3-, it is done this way.

How many moles K^+ are in 1.81 g? That is moles = g/molar mass
moles K^+ = 1.81/39.098 = 0.0463 moles. Divide that by 3 to find moles K3PO4. There is one (1) mole PO4^3- ions in 1 mole K3PO4 and that can be converted to # ions by moles x 6.022E23.