Purpose:
The purpose of this lab is to calculate the acid dissociation constant
(K a
) of acetic acid, HC
2
H
3
O
2
, and verify that it is indeed a constant regardless of the concentration of the acid.
Background:
An acid dissociation constant, K a
(also known as the acid-ionization constant), is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a dissociation chemical reaction in the context of acid-base reactions. The dissociation equation is often written as HA A
−
+
H
+
, where HA is a generic acid that dissociates into A
−
(the conjugate base of the acid) and H
+
, a hydrogen ion that exists in aqueous solutions as a hydronium ion. In this experiment, HA represents acetic acid and A
−
represents the acetate ion: HC
2
H
3
O
2(aq)

H +
(aq)
+ C
2
H
3
O
2(aq)
. The chemical species
HA, A
−
and H
+
are said to be in equilibrium when their concentrations do not change with the passing of time. The dissociation constant is usually written as a quotient of the equilibrium concentrations (in mol/L), denoted by [HA], [A
−
] and [H
+
]:
Materials:

CBL program

TI Graphing Calculator w/
Datamate program

Wash bottle filled with distilled water

(2) Stirring rods





100mL graduated cylinder pH Sensor
1 Pipet
Small beaker
2.00 M HC
2
H
3
O
2
Procedure:
1) Obtain and wear safety goggles.
2) Plug the pH Sensor into Channel 1 of the CBL 2 interface. Use the link cable to connect the TI Graphing Calculator to the interface. Firmly press in the cable ends.
3) Turn on the calculator and start the DATAMATE program. Press
CLEAR to reset the program.
4) Select SETUP from the main screen.
5) Press
ENTER to select
CH 1
.
6) Select PH from the SELECT SENSOR menu, and then select OK to return to the main screen.

7) Pour the calculated volume of acetic acid required for the lab into a small beaker.
8) Pour about 20mL of distilled water into a 100mL graduated cylinder, followed by the calculated amount of acetic acid required to make a the assigned molar solution in the same graduated cylinder. Then add more distilled water to the graduated cylinder until the 100mL mark is reached.
9) Stir the solution in the graduated cylinder thoroughly to prevent pH readings from drifting.
10) Rinse the pH sensor with distilled water, and stick it into the graduated cylinder to measure the pH of the solution.
11) Record the pH when the reading stabilizes.
12) Repeat Steps 8-11 with the 2 nd
assigned concentration of acetic acid.
Safety Precautions:
Handle acetic acid with care - it will cause painful burns if it comes into contact with skin or gets into eyes.
Analysis:
The K a
expression was determined by dividing the product of the concentrations of the products in the reaction by the product of the concentrations of the reactants. The volumes of 2M acetic acid used were
20mL and 25mL, which were calculated by dividing 0.400M, the assigned concentration, by 2.00M, the concentration of the acetic acid, and then
0.500M, the other assigned concentration, by 2.00M. The [H
+
] values of the two solutions were calculated by taking the antilog of the measured pH values because pH is the –log[H
+
]. The [C
2
H
3
O
2
] values of the two solutions were the same as the [H
+
] values of each respective concentration because there is 1:1 stoichiometric ratio between the two products. The [HC
2
H
3
O
2
] values of the two solutions were calculated by subtracting the [H + ] or [C
2
H
3
O
2
] values from the assigned concentrations, respectively, to find out how much how much
HC
2
H
3
O
2 was still remained after the reaction. The K a calculations were done using the K a
expression for acetic acid, [C
2
H
3
O
2
] [H + ]
---------------------------
.
[HC
2
H
3
O
2
]
Possible sources of error include not measuring solution amounts precisely, an inaccurate pH sensor, and human discretion when deciding when the reading was “stabilized” (because the reading never completely stabilized) and the pH should be recorded.

Observations:
The acetic acid had a pungent odor. When the solution in the graduated cylinder was mixed, the acid was suspended for a brief moment. The displayed pH readings on the calculator screen fluctuated greatly.
Conclusion:
The K a
of acetic acid was experimentally determined to be
1.325 X 10 -5 , and was indeed a constant regardless of the concentration of the acid. The K a
of same which was calculated by taking the average of the K a
of the 0.400M and the 0.500M acetic acid solutions. The equilibrium constant expression, K a
, for the dissociation of acetic acid allowed the calculation of K a by formulating a mathematical relationship in which K a
equals the product of the concentrations of the products in the reaction by the reactants in the chemical reaction.

Jonathan Chen
AP Chemistry/4 th
pd./1-5-10
Acid Dissociation Constant, K a
Pre-Lab
1) Write the equilibrium constant expression, K a
, for the dissociation of acetic acid, HC
2
H
3
O
2
.
K a
= [C
2
H
3
O
2
] - [H] +
---------------------------
[HC
2
H
3
O
2
]
2) You have been assigned two different concentrations of acetic acid solution by your teacher. Determine the volume, in mL, of 2.00 M acetic acid required to prepare these two solutions.
0.400M/2.00M = 20.0mL
0.500M/2.00M = 25.0mL