Question

A scientist adds 1.00 moles of phosphoric acid to a liter of water to make a solution.
How many moles of NaOH should be added to ensure that the concentration of H2PO4- is equal to three times the concentration of HPO42? Remember that H3PO4 is a triprotic
acid.

I don't even know where to start with this question. Please help.

Answer 1

Here is what I would do. First, add 1 mol NaOH to convert ALL of the H3PO4 to H2PO4^- ion. Keep that 1 mol NaOH in mind.

Now look at how we convert H2PO4^- to H2PO4^=
......H2PO4^- + OH^- ==> HPO4^=
I.....1 mol.....0.........0
add.............x...........
C....-x........-x..........x
E....1-x........0..........x

Now you want 1-x to equal 3x
1-x = 3x
x = 0.25

So you want to add 1 mol to convert all of the H3PO4 to H2PO4^- and another 0.25 mol to convert H2PO4^- and make it 3x the HPO4^2-. That is 1.25 mols NaOH total. Interesting problem. eh?