Question

Which of the following can not act as buffer solution -

• A. 30 ml 0.1M $CH_3COOH$ + 10 ml 0.3M $NH_4OH$
• B. 10 ml 0.2M $Na_2CO_3$ + 20 ml 0.2M $H_2CO_3$
• C. 30 ml 0.1M HCl + 10 ml 0.1M $Na_3PO_4$

Answer 1

Answer: (C)

C

Answer 2

A buffer solution resists changes in pH upon addition of small amounts of acid or base. It typically consists of a weak acid and its conjugate base, or a weak base and its conjugate acid, in significant amounts.

Let's analyze each option:

(A) 30 ml 0.1M $CH_3COOH$ + 10 ml 0.3M $NH_4OH$

Moles of $CH_3COOH = 30 \times 0.1 = 3$ mmol Moles of $NH_4OH = 10 \times 0.3 = 3$ mmol

$CH_3COOH$ is a weak acid and $NH_4OH$ is a weak base. They react to form the salt $CH_3COONH_4$:

$CH_3COOH + NH_4OH \rightleftharpoons CH_3COONH_4 + H_2O$

Since equal moles of weak acid and weak base are mixed, the reaction will proceed to form the salt. At equilibrium, there will be a significant amount of $CH_3COONH_4$ and small amounts of unreacted $CH_3COOH$ and $NH_4OH$ due to the equilibrium nature of the reaction. A solution of a salt of a weak acid and a weak base can act as a buffer. For example, adding a strong acid will react with the acetate ion ($CH_3COO^-$) produced by the dissociation/hydrolysis of the salt, and with $NH_4OH$. Adding a strong base will react with the ammonium ion ($NH_4^+$) and $CH_3COOH$. So, this mixture can act as a buffer.

(B) 10 ml 0.2M $Na_2CO_3$ + 20 ml 0.2M $H_2CO_3$

Moles of $Na_2CO_3 = 10 \times 0.2 = 2$ mmol. $Na_2CO_3$ provides $CO_3^{2-}$ ions (2 mmol). Moles of $H_2CO_3 = 20 \times 0.2 = 4$ mmol.

$H_2CO_3$ is a weak acid and $CO_3^{2-}$ is the conjugate base of $HCO_3^-$. In the carbonic acid system, $H_2CO_3$ and $HCO_3^-$ form a buffer, and $HCO_3^-$ and $CO_3^{2-}$ form another buffer. The reaction between $H_2CO_3$ and $CO_3^{2-}$ is:

$H_2CO_3 + CO_3^{2-} \rightleftharpoons 2HCO_3^-$

Initial: 4 mmol 2 mmol 0 mmol Change: -2 mmol -2 mmol +4 mmol (since $CO_3^{2-}$ is limiting) Final: 2 mmol 0 mmol 4 mmol

The final solution contains 2 mmol of $H_2CO_3$ (weak acid) and 4 mmol of $HCO_3^-$ (its conjugate base). This is a mixture of a weak acid and its conjugate base, which is a buffer solution.

(C) 30 ml 0.1M HCl + 10 ml 0.1M $Na_3PO_4$

Moles of HCl = $30 \times 0.1 = 3$ mmol. HCl is a strong acid. Moles of $Na_3PO_4 = 10 \times 0.1 = 1$ mmol. $Na_3PO_4$ provides $PO_4^{3-}$ ions (1 mmol).

HCl is a strong acid and will react with the basic anion $PO_4^{3-}$. The protonation of $PO_4^{3-}$ occurs stepwise:

$PO_4^{3-} + HCl \rightarrow HPO_4^{2-} + Cl^-$ Initial: 1 mmol 3 mmol 0 mmol After reaction 1: 0 mmol 2 mmol 1 mmol ($HPO_4^{2-}$ formed)

$HPO_4^{2-} + HCl \rightarrow H_2PO_4^{-} + Cl^-$ Initial: 1 mmol 2 mmol 0 mmol After reaction 2: 0 mmol 1 mmol 1 mmol ($H_2PO_4^{-}$ formed)

$H_2PO_4^{-} + HCl \rightarrow H_3PO_4 + Cl^-$ Initial: 1 mmol 1 mmol 0 mmol After reaction 3: 0 mmol 0 mmol 1 mmol ($H_3PO_4$ formed)

The final solution contains 1 mmol of $H_3PO_4$ (a weak acid) and salts (NaCl). It does not contain a significant amount of $H_3PO_4$ and its conjugate base ($H_2PO_4^-$), or any other conjugate acid-base pair of the phosphoric acid system in significant amounts. A solution containing only a weak acid is not a buffer solution.

Based on the analysis, mixture (C) cannot act as a buffer solution.