Kaplan MCAT General Chemistry Review

You can think of buffer systems as kitchen sponges that soak up strong acids and strong bases to minimize changes in pH for a solution.

The Henderson-Hasselbalch equation is used to estimate the pH or pOH of a solution in the buffer region where the concentrations of the species and its conjugate are present in approximately equal concentrations. For a weak acid buffer solution,

Key Concept

The Henderson-Hasselbalch equation is also useful in the creation of buffer solutions other than those formed during the course of a titration. By careful selection of the weak acid (or base) and its salt, a buffer at almost any pH can be produced.

Note that when [conjugate base] = [weak acid] (in a titration, halfway to the equivalent point), the pH = pK_a because log 1 = 0. Buffering capacity is optimal at this pH.

Likewise, for a weak base buffer solution,

and pOH = pK_b when [conjugate acid] = [weak base]. Buffering capacity is optimal at this pOH.

One of the subtleties of buffer systems and Henderson-Hasselbalch calculations that usually goes unnoticed or misunderstood by students is the effect of changing the concentrations of the conjugate pair but not changing the ratio of their concentrations. Clearly, changing the concentrations of the buffer components in such a way that results in a change in their ratio will lead to a change in the pH of the buffer solution. But what about changing the concentrations while maintaining the ratio of the buffer components? For example, what is the effect on the system of doubling the concentrations of the acid and the base (thereby maintaining a constant ratio of the two)? Because we are taking the log of the ratio of the components, the logarithmic value will not change as long as the ratio doesn’t change. If the ratio of the buffer components doesn’t change, the pH of the buffer solution doesn’t change. Nevertheless, something has changed, to be sure. The buffering capacity—the size of that kitchen sponge—has changed. Doubling the concentrations of the buffer components produces a buffer solution with twice the buffering capacity. The kitchen sponge is twice as big and can soak up twice as much acid or base.

Real World

Blood pH is maintained in a relatively small range (slightly above 7) by a bicarbonate buffer system. Too great a change in either direction would lead to acidosis or alkalosis.

Conclusion

In this chapter, we have reviewed the important principles of acid-base chemistry. We clarified the differences among the three definitions of acids and bases, including the nomenclature of some common Arrhenius acids. We investigated important properties of acids and bases, including the important acid-base behavior of water (auto-ionization) and hydrogen ion equilibria. We explained the mathematics of the pH and pOH logarithmic scales and demonstrated a useful Test Day shortcut for approximating the logarithmic value of hydrogen ion or hydroxide ion concentrations. Strong acids and bases are defined as compounds that completely dissociate in aqueous solutions, and weak acids and bases are compounds that dissociate only partially (to an equilibrium state). We discussed neutralization and salt formation upon reaction of acids and bases, and finally, we applied our fundamental understanding of acid-base reactivity to titrations, useful for determining the concentration of a known acid or base solution, and to weak acid and weak base buffers, useful for minimizing changes in pH upon addition of strong acid or base.

Wow! This was a long chapter, packed with a lot of concepts and information. You’ve certainly earned a break after this. You’ve just accomplished a major task in the overall effort to earn points on Test Day. You may not understand everything that you’ve just read, and you probably don’t remember everything—and that’s okay. Now that you’ve read through these concepts, take some time to work through some of your MCAT practice passages and questions related to these topics so that the concepts settle and solidify. If you find that you need to review for a second or third time some of the discussion points related to acid-base chemistry, we’re always here for you in these pages.

If you’ve been paying attention (and we’re sure you have been), you’ve probably noticed that you’re now one chapter away from completing this review of general chemistry. While we don’t want to offer our congratulations prematurely, we want to acknowledge all the hard work you’ve invested in this process. Keep it up: Success on Test Day is within your grasp!

CONCEPTS TO REMEMBER