Matt Marthaler
Quantitative Analysis Lab 7
Comparison of Unknowns: Weak Acid Identification
Purpose:
Within this lab, I will gain a deeper understanding of acid-base chemistry. I will derive the
necessary equations for this lab and use them on my data. At the end of the lab, I will identify the
unknown acid that I have received.
Procedure:
1) Obtain the unknown acid. If the acid is a solid, dry it in the oven for at least one hour.
2) The weak acid will then be titrated by a strong base.
3) We will then use dried potassium acid phthalate (KHP) for the titration. Approximately 0.61289
g KHP will be used diluted in ~50 mL water for a 30 mL titration with NaOH.
4) 4.17mL of 6M NaOH was diluted in 250mL distilled water to prepare approximately 0.1M NaOH.
5) A pH meter was calibrated with buffer solutions of pH 4, 7 and 10.
Weak Acid – Strong Base titration
6) The KHP (weak acid) solution was titrated with about 2mL increments of NaOH (strong base); a
pH reading was obtained after each addition of the base.
7) Increments were decreased to 0.5 mL when pH rapidly increased. 0.5 mL increments were used
until change in pH slowed again.
8) The increments were increased back to 2.0 mL until the pH changes were minimal. This
procedure was repeated for two more trials.
Unknown Acid – Strong Base titration
9) Approx. 0.6129 g of unknown acid (A) was massed and diluted with 50 mL of distilled water in
250 mL Erlenmeyer Flask.
10) The unknown acid solution was titrated with NaOH in the same manner that the KHP was
titrated. This process was repeated for 2 additional trials.
Data:
KHP Titration
Trial
Mass of KHP (g)
Half-equivalence
Equivalence point
point (mL of NaOH
(mL of NaOH
added))
added)
pH equivalence
point
1
0.6113
15 mL
30 mL
9
2
0.6251
15 mL
30 mL
9
3
0.6146
14.5 mL
29 mL
8
KHP Titration
Titration of Unknown A
Trial
Mass(g) Acid
Eq. Point
Half Eq Point
1
0.6035
48 mL
24 mL
2
0.6229
50 mL
3
0.6129
Average
0.6131
MW Acid
pKa
Ka
121.063
4.01
9.77 x 10-5
25 mL
124.995
4.01
9.77 x 10-5
49.5 mL
24.75 mL
122.578
4.01
9.77 x 10-5
50 mL
25 mL
122.879
4.01
9.77 x 10-5
(g/mol)
Unknown Titration
Calculations:
Mass (g) KHP Needed for Titration
. 030 𝐿 𝑡𝑖𝑡𝑟𝑎𝑛𝑡 ×
. 100 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻
1 𝑚𝑜𝑙 𝐾𝐻𝑃 204.23 𝑔 𝐾𝐻𝑃
×
×
= 0.61289 𝑔 𝐾𝐻𝑃
1𝐿
1 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻
1 𝑚𝑜𝑙 𝐾𝐻𝑃
[NaOH]
. 6113 𝑔 𝐾𝐻𝑃 ×
1 𝑚𝑜𝑙 𝐾𝐻𝑃
1 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻
1
×
×
= 0.0997 𝑀 NaOH
204.23 𝑔 𝐾𝐻𝑃 1 𝑚𝑜𝑙 𝐾𝐻𝑃 0.030 𝐿 𝑡𝑖𝑡𝑟𝑎𝑛𝑡
Molar Mass (g/mol) of Unknown Acid
. 050 𝐿 𝑁𝑎𝑂𝐻 ×
0.0997 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻 1 𝑚𝑜𝑙 𝑈𝑛𝑘𝑛𝑜𝑤𝑛 𝐴𝑐𝑖𝑑
×
= 0.0049855 mol Unknown Acid
1𝐿
1 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻
0.6129 𝑔 𝑈𝑛𝑘𝑛𝑜𝑤𝑛 𝑢𝑠𝑒𝑑
= 122.578 g/mol
0.004985 𝑚𝑜𝑙 𝑈𝑛𝑘𝑛𝑜𝑤𝑛
Ka
𝐾𝑎 = 10−𝑝𝐾𝑎 = 10−4.01 = 9.77 × 10−5
Conclusion:
This lab was done to discover the identity of an unknown acid by a series of acid base titrations.
We first did a titration of weak acid KHP by strong acid, NaOH. This was used as a standard and the data
for these three titrations came out well. After this, we again use the NaOH to titrate the unknown acid.
These titrations came out very good. These titrations helped us get the Ka, pKa and molecular weight of
the unknown acid. Once we discovered these numbers, we then narrowed our list of possible
compounds. Our Ka was very similar to a bunch of compounds, so we had to choose our compound off
of molecular weight. Benzoic acid was closest to our calculated weight, so we decided this was our
unknown. Our data was not the same as Benzoic acid, but it was still close. Some errors in our
experiment could have come from not measuring amounts correctly or some mathematical errors.
Overall, our results turned out to get the correct identity, so this lab can be considered a success.