Titration problem

Question

25.0 mL of phosphoric acid (H3PO4) solution with an unknown concentration is titrated with a 0.100M solution of NaOH. It takes 35.42 mL of the NaOH to reach the end-point. What was the concentration in the original phosphoric acid solution?

 

A. 0.141M

B. 0.0472M

C. 0.0157M

D. 21.2

E. 7.06

Answer 1

Write a correctly balanced equation 

H3PO4 + 3NaOH ==> Na3PO4 + 3H2O

moles NaOH needed = 0.03542 L x 0.100 mol/L = 0.003542 moles

moles H3PO4 present = 0.003542 mol NaOH X 1 mol H3PO4/3 mol NaOH = 0.0011807 moles

Concentration of H3PO4 = 0.0011807 mol/ 0.025 L = 0.0472M ANSWER = Choice B