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EXPERIMENT
K a OF ACETIC ACID
INTRODUCTION
A weak acid must be studied in terms of its equilibrium constant in order to determine the
concentration of H 3 O +^ ions in its solution. For example, for the general acid, HX, the equilibrium reaction would be
HX (aq) + H 2 O (l) H 3 O
(aq) +^ X^
and the equilibrium constant expression would be given by
Ka = [ H 3 O+^ ] [ X -^ ] [HX]
Ka is constant at a given temperature and is characteristic of the acid, HX, regardless of the manner in which the acid solution was prepared.
In today’s experiment you will determine the value of the equilibrium constant, Ka , for acetic acid by measuring the pH of the acid solution. Also, you will study the effect of adding an additional amount of one of the ions involved in the equilibrium which according to Le Chatelier’s principle shifts the equilibrium so as to consume some of the added ions.
SAFETY PRECAUTIONS
- Wear safety goggles at all times while in the laboratory.
- The acids and salts to be used are in fairly dilute solutions, but may be irritating to the skin. Wash if they are spilled and inform the instructor.
PROCEDURE CHECK OUT A pH PEN FROM THE STOCKROOM
1. Add about 20 ml of 0.10 M of HC 2 H 3 O 2 into a clean dry labeled small Erlenmeyer flask.
2. Add about 20 ml of 1.0 M HC 2 H 3 O 2 .into a clean dry labeled small beaker.
3. Add about 10 ml of 1.0 M NaC 2 H 3 O 2 into a clean dry labeled test tube
A. COMMON ION EFFECT
- In one well of a spot plate, add a drop of methyl orange indicator to 5.00 ml of 0.10 M
HC 2 H 3 O 2. Record the color.
- Add a few drops of 1.0 M NaC 2 H 3 O 2 to the above solution. Record the color change. You will see the color better if there is a white background behind the spot plate. Put a white sheet of paper under the spot plate.
- Answer the questions on the report sheet.
B. THE IONIZATION CONSTANT, Ka , FOR ACETIC ACID
- At your desk, prepare the following solutions into the wells of a clean dry spot plate
a. Fill 2/3 of a well with 0.10 M HC 2 H 3 O 2. b. Fill 2/3 of a second well with 1.0 M HC 2 H 3 O 2. c. In a third well add 4.00 ml of 1.0 M HC 2 H 3 O 2 and 1.00 ml of 1.0 M NaC 2 H 3 O 2. d. In a fourth well add 2.00 ml of 1.0 M HC 2 H 3 O 2 and 3.00 ml of 1.0 M NaC 2 H 3 O 2. e. In a fifth well , add 1.00 ml of solution (d) above and 4.00 ml distilled H 2 O
- Measure the pH of each of the above solutions using the pH pen. Calculate the H 3 O+ concentration for each solution from the measured pH value.
- For mixtures c, d, and e, calculate the new concentrations of HC 2 H 3 O 2 and C 2 H 3 O 2 -^. Show complete setups, showing any trace sources of ions or any difference between original and equilibrium concentrations.
- Fill in the chart on your report sheet for each of the above solutions. Calculate Ka for each
of the above solutions. How constant is Ka?
- Calculate your average Ka value for acetic acid, and the precision of your result.
B. THE EQUILIBRIUM CONSTANT FOR THE IONIZATION OF ACETIC ACID, Ka
a. 0.1 M HC 2 H 3 O 2 Measured pH= _______________ [H 3 O +] calculated from the measured pH : [H 3 O +] = ___________M Calculation of Ka :
HC2H3O2(aq)^ +^ H2O(l)^ H3O+^ (aq)^ + C2H3O2 -(aq)
Initial Conc.
Change in Conc.
Equi. Conc. Setup: Setup: Setup:
Write the equilibrium constant expression for the above equation.
Ka = ______________
Calculate the numerical value of the ionization constant, Ka.
Setup:
Ka= _________________
b) 1.0 M HC 2 H 3 O 2 Measured pH= _______________ [H 3 O +] calculated from the measured pH : [H 3 O +] = ____________ M
Calculation of Ka :
HC2H3O2(aq)^ +^ H2O(l)^ H3O+^ (aq)^ + C2H3O2 - (aq)
Initial Conc.
Change in Conc.
Equi. Conc. Setup: Setup: Setup:
Write the equilibrium constant expression for the above equation.
Ka = ______________
Calculate the numerical value of the ionization constant, Ka.
Setup:
Ka= _________________
d. 2.00 ml of 1.0 M HC 2 H 3 O 2 and 3.00 ml of 1.0 M NaC 2 H 3 O 2
Measured pH= _______________ [H 3 O +] calculated from the measured pH : [H 3 O +] = ____________ M
Calculate the new concentration of HC2H3O2:
Setup:
MHC 2 H 3 O 2 = _______M
Calculate the new concentration of NaC2H3O2:
Setup:
MC 2 H 3 O 2 - = _______M
Calculation of Ka :
HC2H3O2(aq)^ +^ H2O(l)^ H3O+^ (aq)^ + C2H3O2-^ (aq)
Initial Conc.
Change in Conc.
Equi. Conc. Setup: Setup: Setup:
Write the equilibrium constant expression for the above equation.
Ka = ______________
Calculate the numerical value of the ionization constant, Ka.
Setup:
Ka= _________________
e. 1.00 ml of mixture (d) and 4.00 ml distilled water. Measured pH= _______________
[H 3 O +] calculated from the measured pH :
[H 3 O +] = ____________ M
Calculate the new concentration of HC2H3O2:
Setup:
MHC 2 H 3 O 2 = _______M
Calculate the new concentration of NaC2H3O2:
Setup:
MC 2 H 3 O 2 - = _______M
Calculation of Ka :
HC2H3O2(aq) +^ H2O(l)^ H3O+^ (aq)^ + C2H3O2 - (aq)
Initial Conc.
Change in Conc.
Equi. Conc. Setup: Setup: Setup:
Write the equilibrium constant expression for the above equation. Ka = ______________
Calculate the numerical value of the ionization constant, Ka.
Setup:
Ka= _______________ INSTRUCTOR’S APPROVAL ________________
Within reasonable experimental error, do you think that the Ka for acetic acid is a constant?
______________
Find average experimental value of Ka for acetic acid.
Setup:
Ka(Average) = ________
EXERCISE
- COMMON ION EFFECT
a. Calculate the [H 3 O
] concentration in 0.45 M solution of barbituric acid,
HC4H3N2O3. Ka for barbituric acid is 1.0 x 10-5. Show the complete setup.
[H 3 O +] = ________
b. Predict the effect of adding sodium barbiturate, NaC4H3N2O3, to the acid solution
in (a) above. i) The position of equilibrium will shift to the: ____________ (right ,or left) ii) The [H 3 O
] will: ___________ (increase, or decrease)
c. Calculate [H 3 O +], if 0.25 mole Na C4H3N2O3 is added to a 1.0 liter of the
barbituric acid solution in (a) above. ( Assume there is no substantial volume change upon the addition of NaC4H3N2O3). Show the complete setup.
[H 3 O +] = __________M
Does your result agree with your prediction in (b) above? _______________
2. DILUTION EFFECT ON THE PERCENT IONIZATION OF A WEAK ACID
a. A weak acid, HX, is 1.3 % ionized in 0.20 M solution. What percent of HX is ionized in a 0.030 M solution? Show the complete setup.
percent ionization__________%
b. From your result in (a) above answer the following questions: i) How did the percent of ionization change upon dilution?
(increased, or decreased) ii) How did the [H 3 O +] concentration of the above weak acid change upon dilution?
(increased, or decreased)
