Question

When you exercise, the burning sensation that sometimes occurs in your muscles represents the buildup of lactic acid (HC3H5O3). In a .20M aqueous solution, lactic acid is 2.6% dissociated. What is the value of Ka for this acid?
A.) 4.3x10^-6
B.) 8.3x10^-5
C.) 1.4x10^-4
D.) 5.2x10^-3

Answer 1

Answer:

The Ka for this acid is 1.4 * 10^-4 (option C)

Explanation:

Step 1: Data given

In a 0.20M aqueous solution, lactic acid is 2.6% dissociated.

Step 2: The equation for the dissociation of HAc is:

HAc ⇌ H+ + Ac¯

The Ka expression is:

Ka = ([H+] [Ac¯]) / [HAc]

1) [H+] using the concentration and the percent dissociation:

(0.026) (0.2) = 0.0052 M

2) Calculate [Ac-] and [H+]

[Ac¯] = [H+] = x = 0.0052 M

3) Calculate [HAc]

[HAc] = 0.20M - x ( since x << 0.20, we can assume [HAc]  = 0.20 M

4) Calculate Ka

Ka= [( 0.0052) ( 0.0052)] / 0.20

Ka = 0.0001352 = 1.4 * 10^-4

The Ka for this acid is 1.4 * 10^-4