Chemistry-acid base equilibria

• chromeX
In summary, p-Hydroxybenzoic acid (HOC6H4COOH) is a weak diprotic acid that can be titrated with a 0.0200 M NaOH solution. The first equivalence point is reached after adding 16.24 mL of NaOH. To determine Ka1 and Ka2, the pH values at 8.12 mL and 16.24 mL of NaOH are needed. Using ICE with HA- + H2O -> A2- + H3O+, the concentration of [H3O+] is found to be 10^-7.02 at 16.24 mL, resulting in Ka1 = 2.69 * 10^-5. To
chromeX
p-Hydroxybenzoic acid, HOC6H4COOH, is a weak diprotic acid. A 25.00 mL aliquot of a dilute solution of HOC6H4COOH is titrated with a 0.0200 M NaOH solution. The first equivalence point was reached after 16.24 mL of the NaOH solution was added.
a. If the values of the pH after 8.12 and 16.24 mL of base added were 4.57 and 7.02, respectively determine Ka1 and Ka2 for p-hydroxybenzoic acid.
b. Calculate the pH of the solution at the beginning of the titration.
c. Calculate the pH of the solution at the second equivalent point of the titration.

What things have you tried so far?

Here's what I've tried- the moles of H2A at the beginning is .01624L times 0.0200M. You also need 32.48mL of base to reach 2nd equiv point. the part that trips me up is that HA-is an amphoteric species.
I used ICE with HA- + H20 -> A2- + H3O+, finding the conc. of [H3O+] as 10^-7.02 with .012992M as conc. of HA-. I ended up with 7.02 * 10 ^-13 as my answer for Ka1, but I don't think that's right. I also don't know what to do for Ka2.

i figured out how to get ka1, but I'm still having trouble with ka2.

how i got ka1-
use ICE with H2A + H2O -> HA- + H3O+, using the values for halfway to the first equivalence point.
Ka1=([HA-][H3O+]/[H2A])
[H2A]=[HA-]=.0049. (cancels out), leaving Ka1 = [H3O+] = 10^-4.57=2.69*10^-5.

Any tips for solving Ka2?

found ka2 to be 3.39*10^-10.

1. What is an acid-base equilibrium?

An acid-base equilibrium is a chemical reaction between an acid and a base that results in the formation of a salt and water. In this reaction, the acid donates a proton (H+) to the base, forming a conjugate base, while the base accepts the proton, forming a conjugate acid.

2. How do you calculate the pH of a solution?

The pH of a solution can be calculated using the formula: pH = -log[H+], where [H+] represents the concentration of hydrogen ions in the solution. The pH scale ranges from 0 to 14, with 7 being neutral, values below 7 being acidic, and values above 7 being basic.

3. What is the difference between a strong acid and a weak acid?

A strong acid completely dissociates in water, meaning that all of its molecules break apart into ions. A weak acid, on the other hand, only partially dissociates, resulting in a smaller number of ions in solution. This leads to a difference in the strength of their acidic properties.

4. How does temperature affect acid-base equilibria?

Temperature has an effect on the equilibrium constant (K) of an acid-base reaction. In general, an increase in temperature will favor the endothermic reaction (absorbing heat), while a decrease in temperature will favor the exothermic reaction (releasing heat). This can shift the equilibrium and change the concentration of products and reactants.

5. What are some real-life applications of acid-base equilibria?

Acid-base equilibria have many real-life applications, including in industries such as water treatment, food and beverage production, and pharmaceuticals. They are also important in biological processes, such as digestion and maintaining the body's pH balance. Additionally, acid-base equilibria are crucial in chemical analysis, as they can be used to determine the concentration of certain substances in a solution.

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