Illustrated Guide to Home Chemistry Experiments

Chapter 13 Laboratory: Chemical Equilibrium and Le Chatelier’s Principle 237

10.

oALIpToN ACTIvITIES

If you have time and the required materials, consider

performing these optional activities:

• Graph pH versus concentration of sodium acetate
  to determine whether the change in pH is linear with
  respect to concentration of sodium acetate.


• Repeat the experiment using 2 mL of glacial acetic
  acid per test tube to determine how (or whether)
  the change in pH varies with the concentration of
  acetic acid.


• Calculate the initial concentrations of acetate ion in
  the samples. The acid dissociation constant (pKa) of
  acetic acid is 4.76 at 25°C. Calculate the expected
  pH value for each of the samples. How closely do the
  theoretical values correspond to the observed values?

POCEDURER
1.f you have not already done so, put on your splash I
goggles, gloves, and protective clothing.

2. Label six test tubes A through F.


3. Prepare a concentrated (nearly saturated) solution of
   sodium acetate by adding 4.0 g of anhydrous sodium
   acetate to 7 mL of water in the graduated cylinder and
   then adding water to bring the total solution volume
   to 10 mL. Stir or shake the solution to ensure that the
   sodium acetate dissolves completely. We’ll define this
   concentration of sodium acetate as 100%.


4. Transfer 5.0 mL of the 100% sodium acetate solution to
   test tube A and 5.0 mL to test tube B.


5. Add 5.0 mL of water to test tube B and mix thoroughly.


6. Transfer 5.0 mL of the solution in test tube B to test
   tube C, add 5.0 mL of water to test tube C, and mix
   thoroughly.


7. Repeat these steps until the five test tubes labeled A
   through E contain, respectively, 5.0 mL each of sodium
   acetate solutions of 100%, 50%, 25%, 12.5%, and 6.25%.


8. Transfer 5.0 mL of water to test tube F.


9. Add 1.0 mL of glacial acetic acid to test tube A and mix
   thoroughly.
   Following the directions supplied with your pH meter, test
   the pH of the solution and record the data in Table 13-1.

Repeat steps 9 and 10 for test tubes B through F.

After 15 minutes have passed, retest the pH of each

of the six test tubes. Repeat the test again after 60

minutes. Record the data in Table 13-1.

11.

12.

TABLE 13-1: pH values of acetic acid in sodium acetate solutions

dISpoSAL: All of the solutions used in this laboratory

can safely be flushed down the drain with copious water.

Test tubeConcentrationpH (immediate)pH (15 minutes)pH (60 minutes)

A 100%

B 50%

C 25%

D 12.5%

E 6.25%

F 0.00%