[batson2002]

I need to determine the molar concentration of phosphoric acid. A 10mL sample underwent titration analysis with sodium hydroxide. 6 trials had taken place:

Trial 1 - Used 15.1mL of NaOH, and titrated solution turned pale pink.
Trial 2 - Used 17.3mL of NaOH, and titrated solution turned pink.
Trial 3 - Used 13.3mL of NaOH, and titrated solution was clear.
Trial 4 - Used 15.3mL of NaOH, and titrated solution turned pale pink.
Trial 5 - Used 21.2mL of NaOH, and titrated solution turned dark pink.
Trial 6 - Used 14.9mL of NaOH, and titrated solution turned pale pink.

I had found the average mLs of base to neutralize the acid, this is with the pale pink results. The average base is 15.1mLs

Based on the final equation of: H3PO4 + NaOH —> Na3PO4 + H2O, I know that the ratio of moles is 1:3. Using this with the equation CV=CV, I determined the ratio of concentrations, which is 1mol/L of acid to every 4.53mol/L of base. What would be the next step to determine the concentration of acid? I cannot figure out how to do it

[Borek]

That's not how you do titration, you should titrate to the first noticeable change in color, your spread of volumes makes the result unreliable.

Pink color suggests titration against phenolphthalein, if so, you have not neutralized the acid completely at the end point.

[batson2002]

This was not an actual titration I did; but rather just a question on my homework

[Borek]

OK, then the idea of using just those with pale pink seems OK.

Ratio is not 1:3.

Just follow the stoichiometry: http://www.titrations.info/titration-calculation

[batson2002]

But how would I be able to do it without any mass or molarity?

[Borek]

You are not given titrant concentration? Then you can't do anything.

[David Tan]

H3PO4 is a triprotic acid and has three pKa values. However, the 1st Ka >> 2nd Ka >> 3rd Ka. As a result, we can't usually observe the last equivalence volume. Nevertheless, the first equivalence volume of NaOH used, V1, should be equal to V2-V1. If you know the concentration of NaOH, you can work out the concentration of the acid since the mole ratio of acid to base is 3:1. Hope it helps.