Question

A 50.00 mL solution of 0.0350 M hypochlorous acid (ka = 3.5 x 10^-5) is titrated with a 0.0105 M solution of NaOH as the titrant. What is the pH of the acid solution after 15.00 mL of titrant have been added?

Answer 1

1. pH of the 0.0350 M hypochlorous acid solution is:

$$pH = \frac{1}{2} \left(-\lg K_a - \lg C_a^0\right) = \frac{1}{2} \left(-\lg 3.5 \cdot 10^{-5} - \lg 0.0350\right) = 2.96$$

2. Acid concentration after 15.00 mL of NaOH have been added:

$$C_a = \frac{C_a^0 \cdot V_a - C_t \cdot V_t}{V_a + V_t} = \frac{0.0350 \cdot 50.00 - 0.0105 \cdot 15.00}{50.00 + 15.00} = 0.0245 \, M$$

3. NaClO concentration is:

$$C_s = \frac{C_t \cdot V_t}{V_a + V_t} = \frac{0.0105 \cdot 15.00}{50.00 + 15.00} = 0.0024 \, M$$

5. pH of the acid solution after 15.00 mL of titrant have been added is:

$$pH = -\lg K_a - \lg \frac{C_a}{C_s} = 3.45$$

Answer: 3,45.