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The Ka of Acetic Acid

Provide a method for determining the Ka of acetic acid by titration with NaOH based on the data below.
Show details of the calculations.

Titration #1
4 drops of methyl red indicator (to show endpoint).
Burette fill with 50ml of 0.1 M NaOH.
Titrate by adding 1ml increments of NaOH.
Continue titration past the endpoint to a pH of 11-12.

1. Determine the pH at the point in which the acetate ion concetration equals the acetic acid concentration. (that is where the acetic acid is 50% ionized). Record this pH value. (ANSWER IS pH = 5.0)

2. The pKa of an acid equals the pH of the solution when 50% of the acid has been ionized. What is the pKa of acetic acid? Calculate the Ka as well. Example: pH = pKa + log[A-]/[HA]

3. Look up the pKa and Ka for acetic acid and record them here.(compare your results from #2 above to the standard or accepted pKa and Ka)

4. Look up the accepted value for the Ka acetic acid in a handbook or on the internet. Calculate the percent error between the experimental and accepted values according to:
% error = / experimental Ka - accepted Ka /
accepted Ka * 100%

Solution Preview

1. Determine the pH at the point in which the acetate ion concentration equals the acetic acid concentration. (that is where the acetic acid is 50% ionized). Record this pH value.

The answer is pH = 5.0

When you titrate a weak acid with a strong base, the pH at half-equivalence corresponds to pKa. This is what you determined experimentally by your titration. Therefore, "experimentally" you determined that the pH at the point in which the acetate ion concentration equals the acetic acid content is 5.0. So, now the questions that follow are designed to determine whether or not your experimental value of this pH is close to the actual value.

2. The pKa of an acid equals the pH ...

Solution Summary

This solution computes the Ka of acetic acid by titration with NaOH.

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