Question

In which of the following acid-base titrations, pH is greater than 8 at the equivalence point?
(A)- Acetic acid versus ammonia
(B)- Acetic acid versus sodium hydroxide
(C)- Hydrochloric acid versus ammonia
(D)- Hydrochloric acid versus sodium hydroxide

Answer 1

Hint: A method of quantitative analysis for determining the concentration of an acid or base by exactly neutralizing it with a standard solution of base or acid having known concentration is known as an acid-base titration.

Complete step by step answer:
-To understand the theory behind acid-base titrations, let us consider a general acid-base reaction that is proceeding with a proton acceptor. In water, the proton becomes solvated ${{H}_{3}}{{O}^{+}}$. The water added to the base may lose hydroxide ion or gain hydronium ion. Acid-base reactions are reversible reactions.
$\begin{align}
  & HA+{{H}_{2}}O\to {{H}_{3}}{{O}^{+}}+{{A}^{-}}(base) \\
 & HA+{{H}_{2}}O\to B{{H}^{+}}+O{{H}^{-}}(acid) \\
\end{align}$
where $[{{A}^{-}}]$ is the conjugate base and $B{{H}^{+}}$ is a conjugate acid. Thus we can now say that,

$Acid+Base\rightleftharpoons \text{Conjugate base + Conjugate acid}$
Hence, ${{K}_{A}}=\dfrac{{{[{{H}_{3}}O]}^{-}}{{[A]}^{-}}}{[HA][{{H}_{2}}O]}\text{ ; }{{K}_{B}}=\dfrac{[HB]{{[OH]}^{-}}}{{{[B]}^{-}}}$

Therefore, the ionic product of water is given as,
${{K}_{W}}=\dfrac{{{[H]}^{+}}{{[OH]}^{-}}}{[{{H}_{2}}O]}$

-Before understanding the types of acid-base titrations, we must know the following important terms-
(i) Titrant- A solution whose concentration is known.
(ii) Titrand- Any solution whose solution is to be known.
(iii) Titration curve- A plot between pH and milliliters of titrand showing how pH changes versus milliliters of titrant during an acid-base titration.
(iv) Equivalence point- The point at which just adequate reagent is added which can completely react with a substance.
(v) Buffer solution- A solution that resists changes in pH.

-Following are the types of acid-base titration-
(i) Strong acid-strong base
(ii) Weak acid-strong base
(iii) Strong acid-weak base
(iv) Weak acid-weak base

-Now we will see the combinations of acids and bases and will classify under the type of acid-base titration-
(i) Acetic acid is a weak acid and ammonia is a weak base. So, this reaction will fall in the fourth type of acid-base titration.
(ii) Acetic acid is a weak acid and sodium hydroxide is a strong base. So this reaction will fall in the second type of acid-base titration.
(iii) Hydrochloric acid is a strong acid and ammonia is a weak base. So this reaction will fall in the third type of acid-base titration.
(iv) Hydrochloric acid is a strong acid and sodium hydroxide is a strong base. So this reaction will fall in the first type of acid-base titration.

-In any type of acid-base reaction, we will get four types of point-
(i) Point 1- when no base is added, so the pH of the analyte will be below.
(ii) Point 2- this is the case just before the neutralization point.
(iii) Point 3- this is the case where the equivalence point is just achieved.
(iv) Point 4- this is the case beyond equivalence point.

-Let us now see the above points in detail for different types of acid-base titrations to determine the pH changes in various types of combinations of acid and bases-
(i) Strong acid-strong base
(1) At point 1, since no base is added, and the acid is strong, the pH will be in the range 0 to 3.
(2) At point 2, the recording of pH is done before the neutralization point is achieved.
(3) At point 3, which is at the equivalence point, the number of moles of base added will be equal to the number of moles of acid in the analyte. So the pH will be neutral, which is 7.
(4) At point 4, the number of moles of the base will dominate in the analyte and the pH starts shifting towards basic.

(ii) Weak acid-strong base
(1) At point 1, since no base is added, and the acid is a weak acid, so the pH will be in the range 5 to below 7.
(2) At point 2, the recording of pH is done before the neutralization point is achieved.
(3) At point 3, which is at the equivalence point, the number of moles of base added will be equal to the number of moles of acid in the analyte. Since the acid is a weak acid and the base is strong, the pH will not be neutral here and will be slightly basic$(pH\sim 9)$.
 (4) At point 4, the number of moles of the base will dominate in the analyte and the pH starts shifting towards basic.

(iii) Strong acid-weak base
(1) At point 1, since no base is added, and the acid is strong, the pH will be in the range 0 to 3.
(2) At point 2, the recording of pH is done before the neutralization point is achieved.
(3) At point 3, which is at the equivalence point, the number of moles of base added will be equal to the number of moles of acid in the analyte. Since the acid is a strong acid and the base is weak, the pH will be acidic$(pH\sim 5)$.
(4) At point 4, the pH will be just slightly above 7 and lower than the above two cases.

(iv) Weak acid-weak base
(1) At point 1, since no base is added, and the acid is strong, the pH will be in the range 5 to below 7.
(2) At point 2, the recording of pH is done before the neutralization point is achieved.
(3) At point 3, which is at the equivalence point, there is no steepness in the graph. The lack of any steep change in pH throughout the titration renders titration of a weak base versus weak acid difficult, hence not much information can be extracted from such a curve.
-So, from the above information, we can say that the pH is greater than 8 at the equivalence point in weak acid-strong base titrations.
So, the correct answer is “Option B”.

Note: Acid-base indicators are substances that are themselves acids or bases stable, soluble and change colors, or develop turbidity at certain pH. Acid-base indicators can be broadly classified as-
(i) Phthaleins and sulphophthaleins (Example Phenolphthalein)
(ii) Azo indicators (Example Methyl orange)
(iii) Triphenylmethane indicators (Example Malachite green)
-For strong acid-strong base titrations, phenolphthalein is usually preferred because of its more easily seen color change.
-For weak acid-strong base titrations, phenolphthalein is used as it changes sharply at the equivalence point and would be a good choice.
-For strong acid-weak base titrations, methyl orange is preferred as it will change sharply at the equivalence point.
-For the weak acid-weak base, neither phenolphthalein or methyl orange is suitable because it requires a vertical portion of the curve over two pH units.