Question

Question: Consider the following four titrations

i. 100.0 mL of 0.10 M HCl titrated with 0.10 M NaOH

ii. 100.0 mL of 0.10 M NaOH titrated with 0.10 M HCl

iii. 100.0 mL of 0.10 M titrated with 0.10 M HCl

iv 100.0 mL of 0.10 M HF titrated with 0.10 M NaOH

Rank the titrations in order of

a. Increasing volume of titrant added to reach the equivalence point

b. Increasing pH initially before any titrant has been added

c. Increasing pH at the halfway point in equivalence.

d. Increasing pH at the equivalence point

How would the rankings change if replaced and if ${\mathbf{C}}_{\mathbf{5}}{\mathbf{H}}_{\mathbf{5}}\mathbf{N}$replaced${\mathbf{CH}}_{\mathbf{3}}{\mathbf{NH}}_{\mathbf{2}}$and if${\mathbf{HOC}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}\mathbf{ }\mathbf{replaced}\mathbf{ }\mathbf{HF}\mathbf{?}$

Short answer

All the volumes are the same.

The increasing order of the initial pH is:

$\mathrm{i}<\mathrm{iv}<\mathrm{iii}<\mathrm{ii}$

The increasing order of the pH at the halfway equivalence point is:

$\mathrm{i}<\mathrm{iv}<\mathrm{iii}<\mathrm{ii}$

The increasing order of the pH at the equivalent point will be:

$\mathrm{iii}<\mathrm{ii}=\mathrm{i}<\mathrm{iv}$

Step-by-step solution

Definition of titration curve

The plot of the pH of the analyte solution versus the volume of titrant added as the titration progresses is defined as the titration curve.

Explanations

a. The first titration(i) includes the titration of a strong acid with a strong base. The titrant is the strong base.

The reaction is

$\mathrm{HCl}+\mathrm{NaOH}\to \mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O}$

Similar volumes of acid and base react with each other to get the equivalence point.

The second titration (ii) includes the titration between the same acid and base, but the titrant is the strong acid now. It reacts in the same way as described above, and the volumes that need to be reacted are equal.

In the third titration (iii), the acid and base react together in a 1:1 ratio.

In the fourth titration (iv), the volume required to reach the equivalence point is the volume of acid.

b. First titration(i): The pH of the strong acid should be calculated as the initial pH

Second titration(ii): The pH of the strong base needs to be calculated as the pH

Third titration(iii): The initial pH here is the pH of the weak base

Fourth titration(iv): The pH of the solution will be more than strong acid but less than the weak and strong base.

c. The halfway equivalence point refers to half of the pH value at the equivalence point.

d. First titration(i): The pH at the equivalence point of the titration of strong acid and strong base is 7.

Second titration(ii): It involves a strong acid and a strong base. Again, the pH at an equivalent point will be 7.

Third titration(iii): The pH of the solution at the equivalent point will be just below 7.

Fourth titration(iv): The pH will be just above 7.