A buffer is formed by mixing 50 ml of a 0.12 M solution of lactic acid, HC3H5O3 with 50 ml of a 0.11 M solution of sodium lactate, NaC3H5O3?

Question

1. Write the chemical equation for the equilibrium that is established
2. Write the Ka expression for lactic acid
3. Explain how this solution acts as a buffer if a couple drops of 0.10 M NaOH are added to the solution; if a couple of 0.15 M HCl are added to the solution
4. Calculate the pH of the lactic acid BEFORE the sodium lactate is added
5. Calculate the pH of the original buffer solution

Answer 1

1) The equilibrium reaction is

 

HC3H5O3  <-> H+ + C3H5O3-

 

2) The Ka expression for lactic acid is

 

Ka = [H+][C3H5O3-]/[HC3H5O3]

 

3) To determine how this solution acts as a buffer you first need to determine the pH of the solution. This can be found by setting up an ICE table since lactic acid is a weak acid. The ICE table is

 

     HC3H5O3     <->    H+ +    C3H5O3-

I:   0.12                     0          0.11

C:  -x                        +x        +x

E: 0.12-x                   x           0.11+x

 

By plugging the equilibrium concentrations and the Ka value into the Ka expression you can determine the pH of the solution.  This answers questions 4 and 5.

 

Now when HCl is added, set up the following reaction

C3H5O3-  +  H+  -> HC3H5O3  +  H2O

to calculate the concentrations of C3H5O3-  and HC3H5O3.

 

For the addition of NaOH set up the reaction

HC3H5O3  +  OH-  -> C3H5O3-  and calculate the concentrations of HC3H5O3 and C3H5O3-.

 

In each case you can calculate the pH either by an ICE table or by the Henderson-Hasselbach equation which is given as follows: pH = pKa + log([C3H5O3-]/[HC3H5O3])

If the difference between the new pH and the original pH is very small then the solution acts as a buffer. Buffering capacity is determined by the ratio [C3H5O3-]/[HC3H5O3]. If the ratio remains unchanged when a strong acid or base is added then the solution is an effective buffer. Try to see this for yourself for this question.

 

Hope this answers your questions.