Question

A 5.30 mL sample of an H₃PO₄ solution of unknown concentration is titrated with a 1.100×10^-2 M NaOH solution. A volume of 7.32 mL of the NaOH solution was required to reach the equivalence point. What is the concentration of the unknown H₃PO₄ solution? Express your answer with the appropriate units.

Answer 1

The reaction between H3PO4 and NaOH is as follows

H3PO4 + 3 NaOH ------------------ Na3PO4 + 3 H2O

1 mole      3 mole

NaOH = 1.100 x10^-2M and 7.32mL

number of moles of NaOH = 1.100 x10^-2 x 0.00732L = 0.00008052 moles

number of moles of NaOH = 0.00008052 moles

according to equation

3 moles of NaOH = 1 mole of H3PO4

0.00008052 moles of NaOH = ?

                                         = 0.00008052 x 1 / 3 = 0.00002684 moles

number of moles of H3PO4 = 0.00002684 mole

volume of H3PO4 = 5.30 mL = 0.00530 L

Concentration of the solution = number of moles / volume in L

                                              = 0.00002684 / 0.00530 = 0.005064 mol/L

Concentration of H3PO4 = 0.005064 mol/L = 5.06 x10^-3M

Concentration of H3PO4 = 5.06 x10^-3M.