Question

The pH of a 0.0516 M solution of nitrous acid, HNO2, is 2.34. What is its Ka?

Answer 1

Solution.

Nitrous acid is a very weak acid, so you need to write the following formula:

$$pH = \frac{pKa}{2} - \frac{lg(H+)}{2}$$

$$pKa = 2 \times (pH + \frac{lg(H+)}{2})$$

$$pKa = 2 \times (2.34 + \frac{lg(0.0516)}{2})$$

pKa = 3.39

$$pKa = -lg(Ka)$$

$$Ka = 10^{-pKa}$$

$$Ka = 4.07 \times 10^{-4}$$

Answer:

$$Ka = 4.07 \times 10^{-4}$$