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Determination of an Unknown Substance by TitrationLarry Sepulveda

Objective: To determine the identity and industrial use of an unknown substance and then reproduce the results.

Methods: gravimetric analysis and acid/base titration

Major Findings: - The concentration of the dissolved substance in KOH and

 solutions was found to be 0.09877 with a standard deviation of 32 ppt. - The substance was identified after analysis and reproduction to be acetic acid (H3CCOOH).

What did we know about the starting materials?

We know concentration (a form of density), which is weight versus volume.

Concentrations of different species:[KOH] =mol L-1 = (mass)(molar mass)-1(Lsoln)-1=(4.105g KOH ) (39.10 g/mol)-1 (1 L) = 1.05 M

[Unknown] = 5.873g (60.01 g/mol)-1(1L)-1 = 0.09788 M

[HOAc] = 5.800g (60.01 g/mol)-1(1L)-1 = 0.09651M

What did we know about the finishing materials?

We know that the finishing solution has become more basic according to its change in pH. The higher the pH, the more basic a solution is. We

also know the acid dissociation constant1 which helps us calculate the pH.

Acetic acid dissociation constant (Ka ) and pH Calculation:Ka = [OAc-][H+]/[HOAc] = x2 / (0.09651-x) ≈ x2 / 0.09651

So x = [H+] = (Ka x 0.09651)1/2 = (1.78x10-5 x0.09651)1/2 = 0.001311 M

And therefore pH = -log 0.001311 = 2.88

Using the same concentration of base and acid, reproduction of the experiment with acetic acid should yield similar results for Ka:

Ka = [OAc-][H+]/[HOAc] = [OAc-](10-pH)/[HOAc] = (10-2.88)2 /[HOAc] =1.9x10-5

HOAc pH ≈ unknown pH

But how did we account for error?

See table in “Results” section.

1. Sigmaaldrich.com. 2. Harris, D., Quantitative Chemical Analysis. 2007, p. 122-127.3. Titration Video. http://www.youtube.com/watch?v=g8jdCWC10vQ

Sample Preparation: A KOH solution (~ 0.1 M) was prepared by adding 4.105 g KOH to one L of water. 5.873 g of unknown solution was added one liter of water under the assumption that acetic acid was the substance and would yield a 0.01 molar solution.

Instrumentation: A titration funnel and stand were set up to titrate the unknown. Phenylaline was used as a pH indicator. Procedure: The unknown was titrated with KOH in order to find the equivalence point volume. The same procedure was followed for acetic acid3(see Figure 7-1,2 also see http://www.youtube.com/watch?v=g8jdCWC10vQ for video presentation).

The unknown acid was determined to be acetic acid with an equivalence point pH of about 3.0 and a Ka approximately equal to that of acetic acid (see discussion). Our objective to identify the substance and reproduce the results was therefore met.

Future work includes determining the concentration of acetic acid in unlabeled bottles of vinegar by titration.

Do you know what’s in your salad?

The food industry uses acetic acid to make vinegars. Our assigned unknown substance is suspected to be this acid. Its chemical formula is H3C-COOH.

Why do we suspect it to be acetic acid?

We used gravimetric analysis, a way to inventory where mass ends up in a reaction, to come up with this hypothesis. First an experiment was demonstrated to us using our unknown. Then, with a list of possible reactions, gravimetric analysis calculations were carried out (see the concentration calculation in the discussion section for an example).

How do we verify that it is acetic acid?

Sample Analysis Method: Titration is a method of acid/base analysis. A weak acid or base is treated with a strong base or acid respectively to determine different properties of the weak species.

Acetic acid (or simply, HOAc) reacts with a strong base in the following way:

HOAc + OH- -OAc + H2O

The acetate ion (-OAc) then undergoes the following equilibrium reaction: -OAc + H2 O OHAc + OH-

Every acid has a specific numerical value (Ka) that describes how strong it is. Every acid also has a number (pH) that describes its acidity after titration (equivalence volume, or Veq). If the textbook values match our experimental values, then our substance tests positive for acetic acid.

ABSTRACT

INTRODUCTION

EXPERIMENTAL

Discussion

CONCLUSION

Works Cited

RESULTS

       

0 5 10 15 20 25 300

1

2

3

4

5

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7

Titration of Unknown with KOH

OH Volume (mL)

pH

0 5 10 15 20 25 300

1

2

3

4

5

6

7

Titration of Acetic Acid with KOH

OH Volume (mL)

pH

Substrate pH at Veq

Ka % Error = (Theoretical-Actual) Theoretical

x 100

Unknown 2.88 1.9 x 10-5 6.74 %

Acetic Acid

3.00 1.78 x 10-5 NA