Textbook Problem J.9 part b

[Natallie K 3B]

I am confused on textbook question J.9 part b. I understand that the salt is (NH4)3 PO4, but the net ionic equation in the solution manual confused me. Isn't H3PO4 a weak acid? Why does it dissociate into H+ and PO43-? Shouldn't it just remain as H3PO4 in the reactants because it is a weak base?

[Zach Jaffery 1K]

A weak acid will still donate protons. It does not dissociate completely, so there will be some H3PO4 left at equilibrium, but there will also be some of the salt present. if weak acids/bases didn't dissociate at all, they wouldn't be able to participate in an A/B reaction.

[Megan Lu 3D]

A weak acid will still donate protons. It does not dissociate completely, so there will be some H3PO4 left at equilibrium, but there will also be some of the salt present. if weak acids/bases didn't dissociate at all, they wouldn't be able to participate in an A/B reaction.

Hi Zach, I also had this question, so I wanted to follow up. I get that the weak acids still dissociate, but my impression was that weak acids are written in their non-dissociated form on the reactant side, and dissociated on the product side. For example, in part C, bromous acid (HBrO2), which is a weak acid, is written as a non-dissociated reactant. Could you clarify?

[Katherine_Douglas_1F]

The answer to your question, Megan, is what Zach meant when he said there would be some H3PO4 left at equilibrium. For Acid-Base reactions involving weak acids or weak bases, it is better to imagine the arrow as a double-sided arrow. Some of the H3PO4 molecules will ionize into H+ and PO43-, but some of them will stay in their reactant form of H3PO4. This is why both forms are written on either side of the arrow. The molecule does exist in both forms.