Acid or Base Question

Question

A solution made by adding 1.300 mol of methylamine (CH3NH2) gas to 1.00 L of water contains 0.00936 mol each of methylammonium ions (CH3NH3+) and hydroxide ions (OH-) and 1.291 mol of CH3NH2 molecules.

Classify CH3NH2 as a strong acid, weak acid, weak base, or strong base.

Answer 1

Let's refresh - what makes something an acid or a base? In Bronsted-Lowry Acid-Base Theory, an acid is a species that donates a proton (a hydrogen cation), and a base is a species that accepts a proton. So, let's look at what is going on here:

When this methylamine (CH₃NH₂) is put in water, the majority (1.291 moles of the original 1.300 moles) remains in its original form - meaning, it neither accepts nor receives a proton (H⁺).

Another VERY small percentage of this methylamine in water exists as CH₃NH₃⁺ (0.00936 mol). This is the conjugate acid of methylamine - it has accepted a proton (H⁺) from the water (H₂O). We can see that it has taken the proton from the water by the presence of an equal amount of OH^- molecules (the conjugate base of water).

The methylamine is accepting protons to become a conjugate acid, but only on a very small scale. We would say, then, that methylamine is acting as a base, but it is a very weak base.