Question

The Ka for methanoic acid, the chemical that causes the sting in insect bites, is 1.8 x 10^-4. Calculate the pH of a 0.40 mol/L methanoic acid solution.

Answer 1

Answer on Question #58555 – Chemistry – Other

Task:

The Ka for methanoic acid, the chemical that causes the sting in insect bites, is $1.8 \times 10^{4}$–4. Calculate the pH of a 0.40 mol/L methanoic acid solution.

Solution:

The equation of the dissociation of methanoic acid:

$$HCOOH = H^{+} + COOH^{-}$$

The molar concentration of methanoic acid:

$$C = 0.40 \, \text{mol/l}$$

Find the equilibrium concentration of hydrogen ions by the formula:

$$\left[ H^{+} \right] = \sqrt{K_{a} \times C}$$

Where:

- $K_{a}$ – dissociation constant;

- $C$ – the molar concentration.

We get:

$$\left[ H^{+} \right] = \sqrt{1.8 \times 10^{-4} \times 0.4} = \sqrt{7.2 \times 10^{-5}} = 0.00848.$$

We compute pH by the formula:

$$pH = -\lg \left[ H^{+} \right].$$

PH value (pH) of the solution is numerically equal to the negative decimal logarithm of the hydrogen ion concentration in the solution.

We get:

$$pH = -\lg \left[ 0.00848 \right] = 2.07$$

Answer: $pH = 2.07$.