Question

What is [OH⁻] in a solution of 0.125 M CH₃NH₂ and 0.130 M CH₃NH₃Cl?

a) 1.8 × 10⁻⁵ M
b) 1.5 × 10⁻⁴ M
c) 2.5 × 10⁻⁴ M
d) 3.0 × 10⁻³ M

Answer 1

The concentration of hydroxide ions ([OH-]) in a solution of methylamine (CH3NH2) and methylammonium chloride (CH3NH3Cl) cannot be determined without the base ionization constant (Kb) for CH3NH2, which is not provided in the question. Therefore, an accurate answer cannot be given from the provided options.

Step-by-step explanation:

The student is asking about the concentration of hydroxide ions ([OH−]) in a solution containing both methylamine (CH3NH2) and its conjugate acid, methylammonium chloride (CH3NH3Cl). Methylamine is a weak base, and when it is in solution with its conjugate acid, it establishes an equilibrium that can be represented by the following equation:

• CH3NH2 + H2O ← CH3NH3+ + OH−

To solve for [OH−], we need to look at the base ionization constant (Kb) of CH3NH2, use the given concentrations of CH3NH2 and CH3NH3Cl, and set up an ICE table to calculate the equilibrium concentrations. However, without the Kb value, we cannot calculate the [OH−] accurately.

Instead, we can use the information given in the question to determine that the concentrations provided are similar to the presumed equilibrium concentrations and utilize the formula for the water ionization constant (Kw = [H3O+][OH−]) to estimate [OH−]. Since the correct Kb value is not provided, we cannot solve the problem and provide the correct answer from the given options (a)-(d).