Question

Methylamine (CH₃NH₂) is a weak base with a Kb of 5.6 x 10⁻⁴. A 75 mL sample of 0.500 M. solution of methylamine in water is titrated with a 0.205 M solution of HCl. Calculate the pH of the solution at the given point during the titration.

A. before any acid is added.
pH =

Answer 1

To calculate the initial pH of the solution before any acid is added in the titration of methylamine, we can use the Kb value of methylamine to determine the concentration of hydroxide ions and then calculate the pH using the pH equation. The initial pH of the solution is 11.05.

Step-by-step explanation:

To calculate the initial pH of the solution before any acid is added, we need to consider the ionization of methylamine. Methylamine (CH₃NH₂) acts as a weak base and can react with water to form the methylammonium ion (CH₃NH₃⁺) and hydroxide ion (OH⁻).

The Kb value for methylamine is 5.6 x 10⁻⁴. We can use the Kb expression to calculate the concentration of hydroxide ions (OH⁻) and determine the initial pH:

Kb = [CH₃NH₃⁺][OH⁻] / [CH₃NH₂]

Since the initial concentration of methylamine is 0.500 M, the concentration of hydroxide ions is equal to the Kb value divided by the concentration of methylamine:

[OH⁻] = Kb / [CH₃NH₂] = (5.6 x 10⁻⁴) / 0.500 = 1.12 x 10⁻³ M

Using the pH equation pH = 14 - pOH, we can calculate the pOH:

pOH = -log[OH⁻] = -log(1.12 x 10⁻³) = 2.95

Substituting the pOH value into the pH equation, we find the initial pH of the solution:

pH = 14 - 2.95 = 11.05