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Unformatted text preview: Experiment 16 – Identification of an Unknown Acid
Spring 2010 Name Alec Rubenstein Lab Section 438 Point Summary (See Blackboard for detailed grading rubric)
Superior Excellent Satisfactory Fair Poor Omitted Introduction
•Purpose of Report
•Goals of Experiment
Materials and Methods Results and Discussion
•Description of data
•Data Table Titles
•Discussion of discrepancies
Other Unknown Identification TOTAL POINTS TA Comments/Suggestions:
CHEMISTRY 102L REPORT TEMPLATE
Identification of an Unknown Acid EXPT. 16 Introduction
The purpose of this report is to explore titration analysis in determining the identity of an unknown
acid. The objective of the experiment was to determine the identity of an unknown acid by titrating the
acid with a NaOH base solution and observing a pH vs. volume titrationt plot. The acid’s identity was
determined by finding the acid dissociation constants and molar mass of the unknown sample via the
titration of the 0.0834 M NaOH base solution created in Lab 15: Standardization of a Sodium Hydroxide
Solution through data and graph analysis. The acid dissociation constants and molar mass of the acid
were then used to find the identity of the unknown acid by comparing them to a list of known acids. Materials and Methods
The procedure for this experiment was taken from UNC Chemistry 102 Lab Manual, Experiment
16: Identification of an Unknown Acid.
The pH sensor was first calibrated in Data Studio. The buret was then conditioned and filled with
the 0.0834 M NaOH base solution. Then, 0.3 to 0.4 g of an unknown acid was weighed by difference
into a clean 100 mL beaker. The beaker was then filled with 50 mL of distilled water and a stir bar was
placed inside. The beaker was then placed on a stir plate and allowed to stir for 5 minutes. The tip of the
pH electrode was then rinsed with distilled water and placed in the acid solution. Data Studio was then
started to record the initial pH of the unknown acid.
The unknown acid was then titrated by adding the NaOH base solution from the buret until 0.3 to
0.5 pH changes were seen. After each pH change, the volume of NaOH base solution was manually
entered into Data studio until a final pH of 12.5 was obtained. Results and Discussion
Figures 1 & 2 provide the necessary data to identify the unknown acid. Both figures provide the
initial pH, the volume of NaOH base at the equivalence point which is used to find the molar mass of
the acid, the equivalence points, and half equivalence points. The initial pH of the acid is found to be
3.90 by analyzing Figure 1. The unknown acid is found to be monoprotic because it only has one
equivalence point on the graph. The equivalence point is identified by finding the greatest slope on
the derivative graph. The volume at this equivalence point can be used to find the molecular weight
of the acid using the calculations seen below the graphs. The half-equivalence point is identified in
Figure 2 and is used to find the pKa of the acid because this is the point where the concentration of the
weak acid and its conjugate base are equal. At this point, the pKa value is equivalent to the solutions
pH. Figure 1. Graph of titration curve of the unknown acid pH and the derivative of the unknown acid pH
curve. The titration curve included pH vs. volume of NaOH titrant. This graph demonstrates that the
unknown acid is monoprotic because it contains only one equivalence point. Figure 2. Graph of titration curve of the unknown acid pH and the derivative of the unknown acid pH
curve. The titration curve included pH vs. volume of NaOH titrant. The equivalence point at 21.180 mL
is the point where the quantity of moles of acid and base are equal. This point was found by analyzing the
greatest slope on the derivative graph. At the half-equivalence point at 10.59 mL, the concentration of the
acid and base are equal and thus the pH equals the pKa.
Molar Mass Calculations:
Volume NaOH x 1L
1 mol acid
x ( Molarity Concentration of NaOH Base solution) x
= moles of acid
1 mol NaOH Mass of acid ( g )
= Molar Mass
Moles of acid (mol )
21.180 mL x 1L
1 mol acid
x 0.0834 M x
= 0.360 mol acid
1 mol NaOH 0.360 g
= 203.8 g / mol
0.001766412 mol Table 1. Raw data for determination of the pKa value and molar mass of the unknown acid. Table values
include mass of the unknown acid, equivalence point, half equivalence point, pKa value, molar mass, and
identity of unknown acid Mass of Unknown Acid
Half Equivalence Point
Unknown Acid Identity 0.360 g
Potassium hydrogen phthalate Based on the titration analysis, the identity of the unknown acid was found to be Potassium
hydrogen phthalate. The titration analysis gave a pKa value of 5.1 and a Molar Mass of 203.8.
Potassium hydrogen phthalate’s molar mass of 204.2151 is almost identical to the value from the titration
analysis and its pKa value of 5.41 is also nearly identical to the value found through the titration analysis.
There were several sources of error in this experiment including both random and systematic error.
A main source of systematic error was from discrepancies in the concentration of the NaOH base solution.
This would happen if the NaOH base solution was not made properly or if its concentration was not
determined correctly in Lab 15. If the true concentration of the NaOH base solution differed from the
previously determined concentration of the solution, all data from this lab would be wrong. Another
source of systematic error could have been in the calibration of the pH sensor and manufacturing errors in
the calibration of the buret and the balance.
Some random errors in this lab included discrepancies in the pH sensor. During several
measurements, the pH sensor jumped several pH points and then dropped down several pH points. This
made it difficult to determine when to make a NaOH base volume measurement. Another random error
included discrepancies in measuring the volume reading through the buret.
These systematic and random errors caused discrepancies in finding the pKa value and molar mass
of the unknown acid. Because the errors in this experiment were minor, the identity of the unknown acid
was found on the first guess. ...
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- Fall '08