[Shing]

H3PO4  is titrated with NaOH

H3PO4 dissociates as following:
H3PO4  <--> H+ + H2PO4- {K1 = 1.1 X 10^-2
H2PO4- <--> H+ + HPO42- {K2 = 7.5 X 10^-8
HPO42- <--> H+ + PO43- {K3 = 4.8 X 10^-13

1) Write the equation beween H3PO4 + NaOH between pH of 8.2 and 9.8
2) calculate molar volume of H3PO4 when volume of NaOH required to reach endpoint is 25.45 ml

What I did..
1) I made the titration curve with buffers regions at phs of the 3 K's and
    know that between the ph of 8.2 and 9.8 HPO42- <--> H+ + PO43- dissociation occurs.. so
    i'm thinking that equation will be Na2HPO4 + NaOH <--> H20 + Na3PO4
    Am I right or wrong?

2) should i do moles of acid = moles of base at endpoint??
    How can i find the molar volumne.

Thanks

[Borek]

1) I made the titration curve with buffers regions at phs of the 3 K's and
    know that between the ph of 8.2 and 9.8 HPO42- <--> H+ + PO43- dissociation occurs.. so
    i'm thinking that equation will be Na2HPO4 + NaOH <--> H20 + Na3PO4
    Am I right or wrong?

Check what are concentrations of H₂PO₄^- and HPO₄^2- at pH 8.2 and at pH 9.8 - then compare them.

How can i find the molar volumne.

No idea what the question asks - molar volume is a volume of one mol of the substance