Aqueous Equilibrium: Ka of an Unknown Acid via Titration

In summary, the conversation discusses a problem involving an unknown acid that is dissolved in water and titrated with an unknown molarity solution of NaOH. After titrating, the solution turns pink after adding phenolphthalein and colorless again after adding more of the unknown acid solution. The problem is to find the Ka of the acid, but the initial volume of the acid solution is unknown, making it difficult to solve. Even an AP Chem tutor is stumped by this problem.
  • #1
Svennerson
2
0

Homework Statement


We're given an unknown acid, and dissolve an unknown mass of the acid in an unknown amount of water. We then titrated the solution with an unknown molarity solution of NaOH. After titrating in 10.0 mL of NaOH, we measure a pH of 5. After 34.68 mL of NaOH, we realize we forgot to add phenolphthaelin. Adding it in turns the solution deep pink. Adding in another 2.62 mL of the unknown acid solution turns the solution colorless again. Find the Ka of the acid.

We are absolutely stumped on this one, please help.
 
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  • #2
I can be missing something, but I don't see how to solve without knowing the initial volume of the acid solution.
 
  • #3
I'm an AP Chem tutor, and this stumps me for the same reason.
 

1. What is an aqueous equilibrium?

An aqueous equilibrium is a state in which the rate of a forward reaction is equal to the rate of a reverse reaction, resulting in no net change in the concentrations of the reactants and products.

2. What is Ka and how is it related to aqueous equilibrium?

Ka, or acid dissociation constant, is a measure of the strength of an acid in aqueous solution. It is related to aqueous equilibrium because it represents the equilibrium constant for the dissociation of an acid into its conjugate base and a hydrogen ion (H+).

3. How is the Ka of an unknown acid determined via titration?

In order to determine the Ka of an unknown acid via titration, a known concentration of a strong base is slowly added to the acid until the equivalence point is reached. The volume and concentration of the base are then used to calculate the Ka value of the acid using the Henderson-Hasselbalch equation.

4. What factors can affect the accuracy of the Ka determination via titration?

Several factors can affect the accuracy of the Ka determination via titration, including the purity of the acid, the accuracy of the titration measurements, and the presence of impurities or other substances that may interfere with the reaction.

5. Why is it important to determine the Ka of an unknown acid?

Knowing the Ka of an unknown acid can provide important information about its chemical properties, such as its strength as an acid and its ability to dissociate in solution. This information can also be used to predict the behavior of the acid in various chemical reactions.

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