posted by Fred .
Molarity of NaOH solution = 0.06115
Volume of NaOH used = 15.50 mL
Volume of unknown Sample = 10.00 mL
Find the Moles of NaOH used?
Find the molarity of H3PO4?
To which end point did you titrate; i.e. how many of the H^+ were neutralized. If all three then
H3PO4 + 3NaOH ==> Na3PO4 + 3H2O
mols NaOH = M x L = ?
mols H3PO4 = 1/3 that.
M H3PO4 = mols H3PO4/L H3PO4
Frankly, I doubt you neutralized all three; you will need to adjust the above if less than three.