Hi,I am sending you the Final report. You will find two reports. The only difference is in the units. I the first one units were written as 22 g/mol NaOH, while in the second one the units were written g/mol 22 NaOH (I realized that your supervisor expects to write like that).Text me if you need some edits .)

DETERMINATION OF Ka FOR AN UNKNOWN WEAK ACID
CHMY 141-0XX
TA: I. Heart Chemistry
**/**/18
INTRODUCTION:
In this experiment, the determination of Ka for an unknown weak acid was carried out. The
determination was based on titration techniques by using a strong base, NaOH. The pKa is characteristic
of the particular acid and can be used for determination of unknown acid. Also, the determination of Ka
is important for expressing the strength of the acid. So the main purpose of the experiment was to
determine the Ka of an unknown acid. The strength of the acid is directly connected to its solubility in
water which is important for possible application in pharmacy, industry, etc.

PROCEDURES AND OBSERVATIONS:

The determination was based on titration techniques to determine the Ka for a solid week acid. Into the
volumetric flask, it was measured 1.075 g of the evaluated solid week acid. The molar mass of the
substance is 204.2 g/mol. In the next step the solid was dissolved with 25 mL of deionized water into the
volumetric flask (100 mL) and finally filled up to the mark with deionized water.
In the cleaned and dried a 100 mL beaker it was pipette 20 mL of HA by using two 10 mL portions. Two
drops of the indicator (phenolphthalein) were added. Then, the previously cleaned and dried buret was
filled with the 0.100 M NaOH solution. The purging of the potential presented bubbles in the tip was
done by run same base into the waste beaker. The Starting Volume of buret was recorded. After the
adding of 4 mL of NaOH solution, the exact volume was recorded to the nearest 0.02 mL which
represented the Final Volume for the data table. The pH was measured using the pH indicator. The
experiment was repeated 5 more times by subsequent addition of a base in 2 ml increments. For each
experiment, the Starting Volume, Final Volume and the pH were recorded and are given in Table 1.

DATA
The data were obtained during the process of titration of unknown solid acid. The titration was based on
the application of a strong base. The measurements of pH were done using the pH probe. In Table 1 it is
presented the results obtained in this study. The average value of pH was 4.72. It the table it was given
the amounts of NaOH added before each pH measurement.

Table 1. The results obtained during the pKa determination for an unknown weak acid
Trial

pH

Starting
Volume (mL)

Final Volume
(mL)

1
2
3
4

4.246
4.494
4.859
5.31

1.24
1.24
1.24
1.24

5.31
7.19
9.19
11.07

Amount
added
(mL)
4.02
5.4
7.4
9.8

pKa

Ka

5.65
5.50
5.58
6.01

2.25x10-6
3.152.25x10-6
2.632.25x10-6
9.792.25x10-7

DATA ANALYSIS AND CALCULATIONS

The Ka was calculated taking into consideration the pH value. The reaction between weak acid (HAc) and
the strong base (OH-) (acid-base) neutralization was used for pKa calculation by using the HendersonHasselbalch equation (equation 1). Furthermore, by using equation 2 it was calculated the Ka of the
unknown solid acid.
[𝐴− ]

𝑝𝐻 = 𝑝𝐾𝑎 + 𝑙𝑜𝑔 [𝐻𝐴]

(1)

𝑝𝐾𝑎 = − log 𝐾𝑎

(2)

The following calculations were used for [A-] and [HA] concentrations calculation after each addition of
NaOH. Those values were used for pKa calculations. The calculation was given only for the first trial as
the same calculations were done for the other trials. The molarity of the acid solution before adding the
reactant (NaOH) was calculated to be 0.05 mol/dm3 by using equation 3.

m (HA) = 1.075 g
M (HA) = 204.2 g/mol
n (HA) = m (HA) / M (HA) = 0.005 mol
V (HA + deionized water) = 100 mL = 0.1 L
[HA] = n (HA) / V (HA + deionized water) = 0.05 mol/dm3
The morality of [A-] was calculated by using equation 4.
1st NaOH addition

(3)

Molarity (NaOH) = 0.01 mol/L
V (NaOH) = 4.02 ml= 4.02x10-3 L
n (NaOH) = 0.01 mol/L x 4.02x10-3 L =4.02x10-5 mol
n (NaOH) = n (A-) = 4.02x10-5 mol
V total = 20 ml + 4.02 mL = 24.02 mL = 24.02 X 10-3 L
[A-] = 4.02x10-5 mol /24.02 X 10-3 L = 1.67 x10-3 mol/dm3

(4)

The molarity of the acid solution after adding the reactant (NaOH) was calculated by using equation 5.

n (HA) = molarity (HA) x V (sample) = 0.05 mol/dm3 x 20x10-3 L=1.05 x10-3 mol
[HA] = (n (HA) – n (NaOH)) / Vtotal
= (1.05 x10-3 mol-4.02x10-5 mol)/ 24.02 X 10-3 L =3.26 x 10-2 mol/dm3

(5)

pKa =pH- (log [A-]/[HA]) = 4.246 – (log ([1.67 x10-3 mol/dm3]/[ 3.26 x 10-2 mol/dm3]))= 5.65
Ka = 10 (-pKa)= 2.25x10-6
It is important mentioning that the phenolphthalein indicator has changed the color to purple when
exposed to acid solutions (reached the equivalence point).
The obtained pKa value was compared with the literature published data [1]. According to the published
data, citric acid has a pKa value of 5.40. Based on the reported values the percentage error was
calculated (equation 6):
%𝑒𝑟𝑟𝑜𝑟 =

𝑝𝐾𝑎𝑡ℎ𝑖𝑠 𝑠𝑡𝑢𝑑𝑦 − 𝑝𝐾𝑎𝑙𝑖𝑡𝑒𝑟𝑎𝑡𝑢𝑟𝑒
5.68 − 5.40
× 100% =
× 100% = 5.2%
𝑝𝐾𝑎𝑙𝑖𝑡𝑒𝑟𝑎𝑡𝑢𝑟𝑒
5.40

The possible error could be related to the measurement uncertainty of pH as this value play an
important role in pKa evaluation

CONCLUSION
The proposed t...