(Chemistry) Ka of unknown acid by titrations.

In summary, to find the Ka of an unknown weak acid, you can use the Henderson-Hasselbalch pH formula and the half-titration data of the acid solution. Since the weak acid is monoprotic, the half-titration gives an equal concentration of acid and conjugate base. From the Henderson-Hasselbalch equation, the pH of the solution is equal to the pKa. Therefore, the Ka can be calculated by taking the inverse log of the pH, which is equal to the concentration of [H+]. However, it is also possible to calculate the Ka using the concentrations of [H+], [A-] and [HA] from the pH and half-titration data.
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Homework Statement


Let's say that i have a unknown weak acid.
  • I prepare 10mL of the weak acid diluted in 25mL of water.
  • I titrate the solution with NaOH. 7.37mL of NaOH was needed for a full titration.
  • Therefore, half-titration of this acid was 3.69mL.
  • I measure the pH of the solution with a pH-meter and the pH was 4.7.

I want to find the Ka of the unknown acid.

Homework Equations


Henderson-Hasselbalch pH formula for a buffer solution is:
pH = pKa + log([base] / [acid])

The Attempt at a Solution


Since the weak acid is monoprotic, the half titration of the solution gives an equal concentration of both acid and conjugate base.

pH = pKa + log(1) = pKa

Does it mean that Ka is the concentration of [H+]?
Ka = 10^-pH = 10^-4.7 = 2.0 e-5?

Edit: Is it possible to calculate in a different way using the given data above?
Ka = [H+][A-]/[HA]
I can calculate the concentration of [H+] from the pH but i don't know the initial concentration of [HA]. And what about the concentration of [A-] ion?
 
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Yes.
 

1. What is the purpose of determining the Ka of an unknown acid by titrations?

The Ka, or acid dissociation constant, is a measure of the strength of an acid. Knowing the Ka of an unknown acid allows scientists to understand its properties and behavior in various chemical reactions.

2. How is the Ka of an unknown acid determined by titrations?

Titrations involve adding a known concentration of a base solution to an unknown acid solution until the equivalence point is reached. The volume of base solution used and the initial concentration of the acid can be used to calculate the Ka value.

3. What equipment is needed for determining the Ka of an unknown acid by titrations?

Some of the equipment needed includes a burette, pipette, standardized base solution, and indicator. A pH meter may also be used to accurately determine the equivalence point.

4. How does the choice of indicator affect the accuracy of Ka determination?

The choice of indicator depends on the pH range of the equivalence point. If the indicator's color change occurs at a similar pH as the equivalence point, the Ka value will be more accurate. Using multiple indicators can also improve accuracy.

5. Can the Ka of an unknown acid be determined without using titrations?

Yes, the Ka value can also be determined through other methods such as spectrophotometry or pH measurement of a buffer solution made with the unknown acid. However, titrations are often the preferred method due to their simplicity and accuracy.

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