CHM 2046C    Sample Module 12   Name: ___Answers_____

 

Part B.  Controlling pH: Buffer Solutions     10 points

 

1.     What is a buffer solution?

 

A buffer solution is a solution resistant to change in pH when an acid or a base is added to the solution.

 

Buffer solutions are simply an example of the common-ion effect; they consist of a weak acid and its conjugate base (the common ion); or a weak base and its conjugate acid, in solution, preferably in equimolar quantities

 

  

2. (a)  State the Henderson-Hasselbalch equation for an acid:

 

                            [conjugate base]

pH = pKa + log (-------------------------)

                                  [acid]

 

  2. (b) State the Henderson-Hasselbalch equation for a base:

 

                               [conjugate acid]

pOH = pKb + log (-------------------------)

                                  [base]

 

3. According to the Henderson-Hasselbalch equation, there are two terms that establish the pH of a buffer solution. What are they, and what are the relative magnitudes of their contribution to the value of the pH?

 

The first: pKa     The Second :

                                         [conjugate base]

                                 log (-------------------------)

                                                 [acid]

The first term establishes the order of magnitude of the [H3O1+] of the buffer solution, i.e. the approximate pH.

 

In the second term the relative concentrations of the weak acid and its conjugate base are adjusted to obtain the exact pH required.

 

In order to maintain a reasonable buffer capacity in both directions (addition of acid and addition of base) the concentrations of the weak and its conjugate base should not differ by more than 10:1 or 1:10

 

4.  What are the two requirements for a buffer solution?

 

(a) The buffer should have sufficient capacity to maintain the pH after addition of anticipated amounts of acid or base

 

(b) It should maintain the pH at the desired level

 

5. Explain how it is that the pH of a buffer solution does not change if the solution is diluted.

 

The second term in the Henderson-Hasselbalch equation contains the ratio of the concentration of the conjugate base of a weak acid to the concentration of the weak acid. If the solution is diluted, then both concentrations change by the same factor, and the ratio of their concentrations remain the same. Therefore the value of the second term  in the Henderson-Hasselbalch equation remains the same and the pH stays the same.

 

 

5. Explain what happens if the concentrations of the buffering components are increased so that their proportion maintains the same ratio.

 

The buffer capacity of the solution increases, that is more acid or more base may be added without changing the pH.