SAT Chemistry Q -- When K3PO4 is dissolved..?

Question

When K3PO4 is dissolved in water, why does the solution turn basic?

KOH is strong and H3po4 is weak

If K is the salt of KOH, then K would make KOH with water which would form H+ ions and turn the solution acidic?

I know its wrong, I just dont know why..

If someone could help.. thanks..

Answer 1

H3PO4 has three H's to lose as H+ before forming PO4 3-. The first H+ comes off at once. H3PO4 is a strong acid. But with the second H+, you have that H+ trying to get away from something that will be HPO4=. And by the time the third H+ leaves, there will be a full PO4 3- pulling it back. So consider things the other way around.

K3PO4(aq) + H2O ===> H2PO4- + K+ + OH-

The OH- increases the pH of Na3PO4, rendering the solution alkaline.

Answer 2

KOH doesn't form precisely because KOH is a strong base. It dissociates in water. H20 + K+ --/-> KOH The equilibrium is in the other direction.

HPO42- is a weak acid and in water PO4^3- + H2O --> HPO42− + OH

Answer 3

The molar concentarion has units of mol L-1 (the number of moles per litre of solution) So work out moles. Mass/ molecular weight = 50 g / 110.98 g/mol = 0.450 mol So now you know there is 0.450 mol per 250 mls, you just convert to mol l-1. to do this: Divide by the volume you have (250) and time by the volume that you want to express the answer as(1000 mls = 1 litre - just converting betwwen units effectively): = (0.450 mol / 250 ml) x 1000 ml = 1.8 mol L-1