Question

The following drawings represent solutions at various stages of the titration of a weak acid, HA, with NaOH. (The Na+ ions and water molecules have been omitted for clarity.) To which of the following regions of the titration curve does each drawing correspond?

A) (i)

1. before addition of NaOH
2. after addition of NaOH but before equivalence point
3. at equivalence point
4. after equivalence point

B) (ii)

1. before addition of NaOH

2. after addition of NaOH but before equivalence point

3. at equivalence point

4. after equivalence point

C) (iii)

1. before addition of NaOH

2. after addition of NaOH but before equivalence point

3. at equivalence point

4. after equivalence point

D) (iv)

1. before addition of NaOH

2. after addition of NaOH but before equivalence point

3. at equivalence point

4. after equivalence point

Answer 1

Concepts and reason

Titration is a quantitative analytical technique used to measure the unknown concentration of a known analyte by reacting it slowly with a substance of known concentration.

Measured volume of the concentration known substance was taken in a conical flask with few drops of indicator, to which the material of unknown concentration was added slowly. At certain point color of the indicator change, this infers the completion of reaction and this is called equivalence point. Concentration of the analyte can be determined from the volume of the analyte consumed for attaining the equivalence point.

Generally concentration of acids and bases were determined by this method. Titration curve is a graph plotted between the volumes of acid or base Vs pH. This is much useful to identify the equivalence of the reaction.

Fundamentals

According to Arrhenius concept, acid is a substance which ionizes in water to produce ${{\rm{H}}^{\rm{ + }}}$ , while base is a substance which ionizes in water to produce ${\rm{O}}{{\rm{H}}^{\rm{ - }}}$ . Acid which readily liberates ${{\rm{H}}^{\rm{ + }}}$ are known as strong acids, while acids which slowly release ${{\rm{H}}^{\rm{ + }}}$ are known as weak acids. The same condition in terms of ${\rm{O}}{{\rm{H}}^{\rm{ - }}}$ is applicable for bases. Acid reacts with base to produce water is called neutralization, which is given below. This is the fundamental for acid-base titrations.

${\rm{HA + O}}{{\rm{H}}^{\rm{ - }}}\, \to \,{{\rm{H}}_{\rm{2}}}{\rm{O}}\,{\rm{ + }}\,{{\rm{A}}^{\rm{ - }}}$

(A.i)

Wrong representations for titration picture ${\rm{i}}$ are given below:

1. before the addition of ${\rm{NaOH}}$ .

3. at equivalence point.

4. after equivalence point.

The correct answer for the picture ${\rm{i}}$ representing the titration of weak acid ${\rm{HA}}$ with ${\rm{O}}{{\rm{H}}^{\rm{ - }}}$ is given below:

2.after addition of ${\rm{NaOH}}$ but before the equivalence point.

(B.ii)

Wrong representations for titration picture ${\rm{ii}}$ are given below:

1. before the addition of ${\rm{NaOH}}$ .

2. after addition of ${\rm{NaOH}}$ but before the equivalence point

3. at equivalence point.

The correct representation for picture ${\rm{ii}}$ is given below:

4. after equivalence point.

(C.iii)

Wrong representation for given titration picture ${\rm{iii}}$ is given below:

2. after addition of ${\rm{NaOH}}$ but before the equivalence point.

3. at equivalence point.

4. after equivalence point.

The correct answer for the picture ${\rm{iii}}$ representing the titration of weak acid ${\rm{HA}}$ with ${\rm{O}}{{\rm{H}}^{\rm{ - }}}$ is given below:

1.before the addition of ${\rm{NaOH}}$ .

(D.iv)

Wrong representation for given titration picture ${\rm{iv}}$ was given below:

1. before the addition of ${\rm{NaOH}}$ .

2. after addition of ${\rm{NaOH}}$ but before the equivalence point

4. after equivalence point.

The correct answer for the picture ${\rm{iv}}$ representing the titration of weak acid ${\rm{HA}}$ with ${\rm{O}}{{\rm{H}}^{\rm{ - }}}$ is given below:

2.at equivalence point.

Ans: Part A.i

The titration of weak acid ${\bf{(HA)}}$ with ${\bf{NaOH}}$ was given in as picture ${\bf{i}}$ , the correct representation of the picture among the set of choices was identified and given below.

$\begin{array}{l}\\{\bf{1}}{\bf{. before the addition of}}\,{\bf{NaOH}}{\bf{.}}\\\\{\bf{2}}{\bf{. after addition of}}\,{\bf{NaOH\;but before the equivalence point}}{\bf{. }}\\\\{\bf{3}}{\bf{. at equivalence point}}{\bf{. }}\\\\{\bf{4}}{\bf{. after equivalence point}}{\bf{.}}\\\end{array}$