Aqueous Ammonia or Ammonium Hydroxide? Identifying a Base as Strong or Weak

Abstract

When grocery stores sell solutions of ammonia, they are labeled “ammonia”; however, when the same solution is purchased from chemical supply stores, they are labeled “ammonium hydroxide”. The goal of this experiment is for students to determine which name is more appropriate. In this experiment, students use several different experimental methods to determine whether these solutions contain more neutral ammonia (and water) molecules or more ammonium and hydroxide ions. This question can be answered using pH or electrical conductivity measurements, but both methods require students to know the solution concentration (determined by titrations). For the pH data, students can make direct comparisons of the pH of the solution with the theoretical pH (assuming ammonia is a strong base), they can calculate a base-ionization constant, and they can calculate the percent ionization of the ammonia solution. For the conductivity data, students compare the conductivities of a hydroxide-containing compound that does not ionize (methanol), one that ionizes completely (sodium hydroxide), and the NH3/NH4OH solution. They can also calculate the percent ionization for the solution using the conductivity data. These comparison methods all show that there are more neutral ammonia (and water) molecules present in the solution than ammonium and hydroxide ions, and these solutions are more accurately called aqueous ammonia and not ammonium hydroxide.