KEY2CHEM

Buffer Solutions

An acid-base buffer is a solution that resists changes in pH when a strong acid or base is added. A buffer is a solution that contains both partners of a weak conjugate acid-base pair
(e.g., $$\require{mhchem}\ce{HA}$$ and $$\require{mhchem}\ce{A-}$$). The two partners disturb one another’s acid-base equilibrium through the common ion effect.

The acidic component of the conjugate pair is capable of reacting with strong base to generate its own weak conjugate base:

$$\require{mhchem}\ce{HA(aq) + OH^{-}(aq) -> A^{-}(aq) + H2O(l)}$$

The weak conjugate base is capable of reacting with strong acid to generate its weak conjugate acid:

$$\require{mhchem}\ce{A^{-}(aq) + H3O+(aq) -> HA(aq) + H2O(l)}$$

Note that each of these reactions goes to completion due to the strong nature of the added acid or base.

Example 1.

A buffer solution contains $$0.50\text{ M } \require{mhchem}\ce{CH3COOH}$$ and $$0.25 \text{ M } \require{mhchem}\ce{CH3COO-}$$. What is true about the [$$\require{mhchem}\ce{CH3COOH}$$] after the addition of $$\require{mhchem}\ce{KOH}$$?

A. $$[\require{mhchem}\ce{CH3COOH}] = 0.50 \text{ M}$$

B. $$[\require{mhchem}\ce{CH3COOH}] > 0.50 \text{ M}$$

C. $$[\require{mhchem}\ce{CH3COOH}] < 0.50 \text{ M}$$

Solution

C. $$[\require{mhchem}\ce{CH3COOH}] < 0.50 \text{ M}$$

$$\require{mhchem}\ce{KOH}$$ is a strong base which will react with the acidic component of the buffer, $$\require{mhchem}\ce{CH3COOH}$$, resulting in a decrease in the $$[\require{mhchem}\ce{CH3COOH}]$$ from its starting value.

Example 2.

After $$\require{mhchem}\ce{HCl}$$ is added to a buffer solution containing equal concentrations of $$\require{mhchem}\ce{NH3}$$ and $$\require{mhchem}\ce{NH4+}$$, which statement is true?

A. $$[\require{mhchem}\ce{NH3}] = [\require{mhchem}\ce{NH4+}]$$

B. $$[\require{mhchem}\ce{NH3}] < [\require{mhchem}\ce{NH4+}]$$

C. $$[\require{mhchem}\ce{NH3}] > [\require{mhchem}\ce{NH4+}]$$

Solution

B. $$[\require{mhchem}\ce{NH3}] < [\require{mhchem}\ce{NH4+}]$$

$$\require{mhchem}\ce{HCl}$$ is a strong acid, which will react with the basic component of the buffer, the $$\require{mhchem}\ce{NH3}$$, to decrease its concentration.

Example 3.

Which is a buffer solution?

A. $$0.50\text{ M HCl } + 0.25\text{ M NaCl}$$

B. $$0.10 \text{ M HF } + 0.25\text{ M KF}$$

C.  $$1.0 \text{ M } \require{mhchem}\ce{HNO3} + 0.25\text{ M } \require{mhchem}\ce{NaNO3}$$

Solution

B. $$0.10 \text{ M HF } + 0.25\text{ M KF}$$

A buffer solution is one that contains both components of a weak conjugate acid-base pair, as the acidic partner will react with added base, and the basic component will react with added acid.