Video Transcript
Which of the following solutions is an example of a weak base? (A) A two-molar solution of NH3. (B) A 0.1-molar solution of NaOH. (C) A 0.2-molar solution of BaOH2. (D) A one-molar solution of LiOH. (E) A 0.5-molar solution of KOH.
In this question, we need to identify which of the solutions listed in the answer choices is an example of a weak base. Each answer choice tells us the molar concentration of the solution and the chemical formula of the solute in the solution. When looking at all of the answer choices, we notice that each solution has a different concentration. Molar concentration is a measure of the amount of moles of a solute dissolved in a specific volume of solution.
Answer choice (B), a 0.1-molar solution of NaOH, has the lowest concentration of all of the answer choices. Answer choice (A), a two-molar solution of NH3, is the most concentrated solution. While we might be tempted to think that the solution in answer choice (A) is a strong base, we’d be incorrect. The molar concentration of a basic solution is not an indicator of base strength. We’ll need to compare the answer choices in a different way.
The strength of a base depends on the ability of the base to ionize. During the ionization process, a molecule or unit of a substance dissociates into separate ions with opposite charges. Strong bases ionize completely in a solution, while weak bases only ionize partially. According to the Arrhenius theory of acids and bases, a base is a substance that produces OH− ions in a solution. When looking at answer choices (B), (C), (D), and (E), we see that the provided chemical formulas contain the hydroxide ion. These are all good examples of Arrhenius bases.
The eight strongest bases are those that are composed of an alkali or alkaline Earth metal and the hydroxide ion. All of the bases in choices (B) through (E) are on this list, so they are strong bases. If we dissolve the base in choice (E), KOH, in water, only the dissolved K+ and OH− ions would be present. No KOH units would remain. The bases in answer choices (B) through (E) are all strong bases; they ionize completely in solution. We can eliminate these answer choices.
Even though the chemical formula of the compound dissolved in the solution in choice (A) does not contain the hydroxide ion, it is still classified as a base. According to the Bronsted–Lowry theory of acids and bases, a base is a substance that accepts H+ ions. Ammonia, or NH3, is an example of a base according to the Bronsted–Lowry theory. In an aqueous solution, the NH3 molecule accepts a hydrogen ion from a water molecule, forming the NH4+ and OH− ions. The equation shown uses an equilibrium arrow because not all of the NH3 molecules ionize. Some weak bases, like ammonia, contain a nitrogen atom with a lone pair of electrons.
The solution that is an example of a weak base is answer choice (A), a two-molar solution of NH3.
