Question

1. write the K_a expression fro the reaction of acetic acid with water.

2. Determine the K_a for a 0.035M weak (monoprotic) acid with a pH of 3.5.

3. This experiment assumes that the pH = pK_a (at the half-neutralization point) of the acid. Why?

(Henderson-Hassel equation)

4. What color will the solution turn indicating that equivalence point has been reached?

5. Define Equivalance Point.

Answer 1

1. Ka expression fro the reaction of acetic acid with water.

Acetic acid dissociates as:

CH3COOH    CH3COO- + H+

Ka = [CH3COO-][H+] / [CH3COOH]

2. Ka for a 0.035M weak (monoprotic) acid with a pH of 3.5.

pH = 3.5

[H+] = 10^-3.5

= 3.162x10^-4 M

Weak acid dissociates as:

HA      A- + H+
Initially 0.035 0 0
Change -x x x
Finally 0.035 - 0.0003162 0.0003162 0.0003162

Ka = [A-][H+] / [HA]

= (0.0003162)(0.0003162) / (0.035 - 0.0003162)

= 9.998x10^-8 / 0.03468

Ka = 2.88x10^-6

3. We know that, at half equivalence point

moles base added = 1/2 * moles of acid originally present

moles of acid left = moles of salt formed = 1/2 * moles of acid originally present

[Salt] = [acid left]

Using Henderson-Hassel equation

pH = pKa + log [salt] / [acid]

Since, [salt] = [acid]

log [salt] / [acid] = log 1 = 0

Therefore,

pH = pKa

5. Equivalence point: The equivalence point, of a chemical reaction is the point at which chemically equivalent quantities of acid and base have been mixed. In other words, the moles of acid are equivalent to the moles of base.