What is the concentration of the phosphoric acid solution of

Question

a 25.00mL sample of the acid requires 42.24mL of 0.135M NaOH neutralization

Answer 1

find moles of NaOH:

0.04224 litres @ 0.135 mol/litre = 5.702e-3 moles of NaOH

@ 1 H3PO4 + 3 NaOH -->  , calculate the moles of H3PO4:

5.702e-3 moles of NaOH @ 1 mol H3PO4 / 3 mol NaOH = 1.90e-3 moles of H3PO4

calculate the Molarity of the 25 ml sample of H3PO4:

1.90e-3 moles  / 0.025 litres = 0.07603 molar

that's four sig figs, unless the "0.135" had more sig figs than listed, it is a 3 sig fig problem:

your answer is: 0.0760 Molar H3PO4