Section: Chemical and Physical Foundations of Biological Systems

28) For a very weak base, the pKb of a solution would likely be:

1. Equal to the pOH
2. Higher than the pOH
3. Lower than the pOH
4. Near 7 at 25ºC

Explanation: The correct answer is B. Normally to calculate pH, pOH, and pKa values, we require some numerical information about the concentration of the solution. In this question that is not given so we will have to go off of the concepts alone. The MCAT often does this and the question is still very doable so do not panic when you find numbers are not provided. Let us write a generic base reaction to represent our molecule:B + H2O <—-> HB+ + OH- Kb = [OH-][HB+]/[B] Using an alternate form of the Henderson-Hasselbalch Equation, expressed in terms of pOH and pKb: pOH = pKb + log [HB+]/[B] Where [HB+] and [B] represent the concentrations of the conjugate acid and original base molecule at equilibrium, respectively. A very weak acid will have a very small Kb. This means that equilibrium favors the reactants [B] over the products [HB+]. Thus, we can infer that the value of [HB+]/[B] << 1, making the log [HB+]/[[B] << 0. This would result in a pOH value that is less than the pKb. Thus, pOH < pKb