Determination Of Ka For A Weak Acids

Melissa Carrithers
AP Chemistry period 6
2-11-15
Determination of Ka of Weak Acids
Purpose:
The purpose of this lab is to find the strength of weak acids by determining the equilibrium constants for their ionization reactions in water. Is to use their measured pH values to calculate the pKa for the two unknown weak acids thus determining their identities.
Hypothesis:
If we neutralize a solution that contains a weak acid by adding a strong base to the solution, then the ions will be isolated and thus finding the concentrations of each species by measuring pH levels and determining [H3O+] possible.
Background:
In order to completely ionize the weak acid, a strong base needs to be added to the solution. When the concentration is determined, the calculation of pH levels will be made possible. Since we know that in this experiment, the molar concentrations of an unknown acid and its conjugate base are equal, it can be determined that the pH of the solutions are equal to the pK(a) of the acid. (pH=-log[H30+] and pK(a)=-logK(a)) A crucial piece of information that should be known is that polyprotic acids contain more than one ionizable hydrogen which occurs through many steps that contain their own equilibrium constant.
Equilibrium Constant Expression for An Acid:
K(a) = [A-][H3O+]/[HA])
Finding K(a): K(a) = [CH3COO-][H3O+]/[CH3COOH] in which acetic acid and acetate ion are equal causing the two to cancel out leaving K(a) = [H3O+]
Procedure
1) Label dishes and record unknown letter for an unknown weak acid.
2) Put small quantities of the unknown into each dish.
3) Measure water and combine the water and sample while stirring.
4) Transfer measured acid solution to a flask and add 3 drops of phenolphthalein and sodium hydroxide solution while stirring.
5) Stir and add until pink color persists in the solution.
6) Mix solution by repeatedly pouring contents into a flask and beaker to thoroughly mix.
7) Measure pH level.
8) Dispose of contents and clean flask.
9) Repeat steps for the second sample.
10) Repeat steps for one of the remaining unknowns.
Data Table:
Unknown
Trial pH pH (average) pKa Unknown Identity A
Sample #1
Sample #2

6.53
6.53
Potassium dihydrogen phosphate B
Sample #1
Sample #2

2.23
2.44

2.34
2.34
Acetylsalicylic acid C
Sample #1
Sample #2

5.94
5.90

5.24
5.24
Potassium hydrogen phthalate D
Sample #1
Sample #2

6.71
6.71
Potassium hydrogen sulfate E
Sample #1
Sample #2

3.19
3.19
Potassium hydrogen tartrate

Post-Lab Questions:

1.) Average the pH readings for each trial to calculate the average pKa value for the unknown weak acids and enter answers in the Data table.

2.) Comment on the precision of the pKa determinations. Describe sources of experimental error and their likely effect on the measured pKa (pH) values.

-The acid not fully dissolving in to the water would result in a no effect pKa value because the NaOH would neutralize the amount of the dissolved acid present.
-Faulty pH meter would cause an unknown effect in the pKa value because it would not properly calibrate.
-Overshooting the endpoint could cause the pKa value to be too high because the base would be too high.

3.) The following table lists the identites of the unknowns in this experiment. Complete the table by calculating the pKa value of each acid.

-Potassium dihydrogen phosphate -log (6.2x10-8) pKa = 7.21
-Potassium hydrogen sulfate –log (1.0x10-2) pKa = 2.0
-Potassium hydrogen phthalate –log (3.9x10-6) pKa = 5.41
-Potassium hydrogen tartrate –log (4.6x10-5) pKa = 4.34
-Acetylsalicylic acid –log (3.2x10-4) pKa = 3.49

4.) Compare the experimental pKa value for each unknown with the literature values reported in question three. Determine the probable identity of each unknown and enter the answers in the Data Table.

5.) Write separate equations for each unknown potassium salt dissolving in water and for the ionization reaction of the weak acid anion that each of these salts contains.

KH2PO4 + H2O ↔ H+ + KHPO4- Potassium dihydrogen phosphate
KHSO4 + H2O ↔ H+ + KSO4- Potassium hydrogen sulfate
KHC8H4O4 + H2O ↔ H+ + KC8H4O4- Potassium hydrogen phthalate
KHC4H4O6 + H2O ↔ H+ + KC8H4O6- Potassium hydrogen tartrate
2-CH3CO2C6H4COOH + H2O ↔ H+ + 2-CH3CO2C6H4COO- Acetylsalicylic acid

6.) Why is it not necessary to know the exact mass of each acid sample?
It is not necessary to know the exact mass of each acid sample because the unknown will dissolve in water and the one important thing, which need to measure is the volume. After the volume from the flask and the beaker mix together they have equal molar.
7.) Why was is it not necessary to know the exact concentration of the sodium hydroxide solution?
It is not necessary to know the exact concentration of sodium hydroxide solutions used because things that need to do is to make the solution reach the equivalence point. There is no need to calculate the equivalence since phenolphthalein is used to indicate when the color changes.
8.) Why was it necessary to measure the exact volume of distilled water used to dissolve the acid, as well as the exact volume of solution transferred from the beaker to the Erlenmeyer flask?
It was necessary to measure the exact volume of distilled water used to dissolve the acid as well as the exact volume of solutin transferred from the beaker to the Erlenmeyer flask because in order to make the solution equal molarity it has to have equal volume. In order to separate the two concentrations in half the volume has to be known.