MCAT Chemistry Question — Weak Base

Fluoride is a very weak base that is added to drinking water at a concentration of 0.7 mg/L. This concentration strengthens the enamel of the tooth surface to prevent the formation of cavities. The pKb of this base would likely be:

A. equal to the pOH of the solution.

B. higher than the pOH of the solution.

C. lower than the pOH of the solution.

D. near 7 at 25ºC.

Click for Explanation

Explanation: The correct answer is B. Normally, to calculate pH or pOH values, we require some numerical information about the concentration of the solution. In this question, this information is not given, so we will have to go off of the concepts alone. The MCAT often requires similar conceptual thinking, and this question is still very doable – so do not panic when you notice that numbers are not provided! Let us write a generic reaction to represent our base in water:

B + H2O <—-> HB⁺ + OH^–

K_b = [OH^–][HB⁺]/[B]

Using an alternate form of the Henderson-Hasselbalch equation, expressed in terms of pOH and pK_b:

where [HB⁺] and [B] represent the concentrations of the conjugate acid and original base molecule at equilibrium, respectively. A very weak base will have a very small K_b. This means that equilibrium favors the reactants (including [B]) over the products (including [HB⁺]). Thus, we can infer that the value of [HB⁺]/[B] << 1, making the value of log [HB⁺]/[[B] << 0. This would result in a pOH value that is less than the pK_b. Thus, pK_b > pOH.

Want more MCAT practice?

We’ve got options for every schedule and learning style!

Whether you need the flexibility of a Self-Paced Course, the instruction of a live 515+ Course, or the 1:1 attention of a private MCAT tutor, Blueprint MCAT has the MCAT prep option that works for your learning style!

Get started with a free MCAT diagnostic, one free practice exam, and tons more MCAT prep resources.

Not sure which option is right for you? Schedule a free MCAT consultation with an MCAT expert using the form below. No obligation, just expert advice.

Schedule My Free Consultation