Question

Explain why using deionized water to rinse the walls of the flask have no effect on the reported results of titrations performed.

Answer 1

The amount of an acid required to neutralize a basic solution depends on the number of moles of the base, assuming that acid is monoprotic and base is monobasic.

Consider, a 100-ml solution of 1 M NaOH:

            Number of moles of NaOH in solution = molarity x volume in L    

                                                            = 1 M x 0.10 L               [1 M = 1 mole /L]

                                                            = (1 moles / L) / 0.1 L

                                                            = 10 moles

Now, consider dilution with distilled water: Rinsing the wall of the flask with distilled/deionized water increases the volume of the solution, thus also reduces it’s concentration. However, the number of moles of NaOH in the flask still remains the same, i.e. 10 moles.

In a reaction 2 HCl + 2 NaOH ------> 2 NaCl + 2 H₂O

It is the number of moles (irrespective of volume of solution) of the acid and bases that react with each other in stoichiometric ratio. Dilution does reduce the concentration, but the number of number of moles of the solute remains constant.

Therefore, dilution/ rinsing the walls with distilled water does not affect the endpoint/ result.