Question

You prepare a diluted NaOH solution by adding 15 mL of 3 M NaOH to 435 mL of distilled water and stir for several minutes. In a clean dry beaker you obtain about 75 mL of standardized HCl and record the molarity. You take two burets, rinse one with the standardized HCl and one with the diluted NaOH solution. Fill one buret with the HCl and the other with NaOH.

Prior to the titration the standard HCl soltion (~15.00 mL) was diluted to a total volume of 65.00 mL.

a. calculate the molarity of HCl in the diluted solution.

b. calculate the normality of HCl in the diluted solution

c. for the titration 5 drops of 0.2% phenolphthalein indicator are added to the beaker containing the ~65 mL HCl solution from question 1. What color is the HCl solution in the beaker?

d. The initial burette reading is 1.45 mL NaOH titrant. The volume of NaOH titrant added to reach the endpoint is 22.57 mL. Calculate the molarity of the NaOH solution.

Answer 1

The initial volume and concentration of NaOH is equated to the final volume and its molarity is found as per the formula V₁M₁ = V₂M₂ and M₂ = (V₁M₁)/V₂ = (15 x 3)/450 = 0.1M.

Phenolphthalein is an acid-base indicator which is a weak acid with ionizable functional groups. In acidic medium, these functional groups remain undissociated and that form of the indicator is colourless. In basic medium, the groups are ionized and in the case given, the sodium salt of the indicator whose anionic form is pink in colour.

It is given that the volume of 0.1M NaOH needed to reach endpoint with HCl is 22.57 - 1.45mL = 21.12mL. The law of equivalence dictates that the concentration of a certain volume of one substance reacting completely with certain volume and concentration of another is related as V_aM_a = VbM_b and M_a = (V_bM_b)/Va with a representing acid and b for base. This gives M_a = (V_bM_b)/V_a = (21.12 x 0.1)/65 = 0.0325M.

In a nutshell a) Molarity of HCl is 0.0325M after dilution.

b) Normality of HCl = 1 x Molarity of HCl = 0.0325N after dilution.

c) The HCl solution with phenolphthalein will be colourless.

d) The concentration of NaOH is 0.1M after dilution.

NOTE: If the answers given are wrong, then concentration of standardized HCl and volume of that HCl taken for titration against NaOH and the volume of NaOH taken for titration against the standardized HCl are required.