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.