Which of the following species has the highest concentration in a solution of K3PO4? (A) K+ aqueous, (B) PO43- aqueous, (C) P5+ aqueous, (D) O2- aqueous, (E) K2PO4- aqueous.
K3PO4 or potassium phosphate is an ionic compound that’s composed of potassium cations and the polyatomic anion phosphate. This means that when we introduce potassium phosphate to water to form an aqueous solution, it will dissociate into potassium ions and phosphate ions. In this problem, we want to know, of our answer choices, which one will have the highest concentration in a solution of potassium phosphate.
Since potassium phosphate will dissociate into potassium and phosphate ions, we know that one of those will be the one that has the highest concentration. So, we can rule out answer choices (C), (D), and (E). So, now, between potassium ions or phosphate ions, which one will have the greatest concentration?
To work this out, let’s assume that we have one liter of a one molar potassium phosphate solution. Since molarity is a unit of concentration that’s equal to moles of our substance divided by the volume of the solution, we can determine the amount of our substance in moles by multiplying the volume by its concentration. Next, we can determine the amount of each of our ions by using the stoichiometric coefficients from our balanced chemical equation.
Since every one mole of potassium phosphate will dissociate to form three moles of potassium ions, we will have three moles of potassium ions in our solution. And the stoichiometric coefficients in front of both potassium phosphate and phosphate are one, meaning that every one mole of potassium phosphate forms one mole of phosphate ions. So, as we can see, one mole of potassium phosphate will dissociate into a greater number of potassium ions than phosphate ions.
And since both ions will be dissolved in the same volume, that means the concentration of potassium ions will be greater than the concentration of phosphate ions in the solution. So, potassium ions will have the highest concentration in a solution of potassium phosphate.