Passage 2: Acid-Base Regulation

Understanding acid-base regulation is often reduced to pigeonholing clinical

states into categories of disorders based on arterial blood sampling. An earlier

ambition to quantitatively explain disorders by measuring the production and

elimination of acid has not become standard clinical practice. Seeking back to

classical physical chemistry, we propose that in any compartment, the requirement

of electroneutrality leads to a strong relationship between charged moieties.

Figure 1 shows the relationship between [H+] of a mixture and the mean [H+] of two

mixtures. Figure 2 shows the equations related to the water dissociation constant.

Strong Relationships in Acid-Base Chemistry – Modeling Protons Based on

Predictable Concentrations of Strong Ions, Total Weak Acid Concentrations, and

pCO2. Adapted from Ring & Kellum (2016).

A solution has 0.1 M acetic acid (Ka= 1.8×10−5 ) and 0.1 M sodium acetate.

Calculate the pH of the solution.

A) pH = 2.3

B) pH = 3.7

C) pH = 4.7

D) pH = 5.3

Click to reveal answer

Correct Answer: C. Since this solution has an acid along with its

conjugate base, it acts as a buffer solution. When thinking buffer solutions, it’s

important to always think of the very important Henderson–Hasselbalch equation,

which is

pH = pKa + log ( [A-] / [HA] )

The MCAT won’t necessarily provide the information that a certain system follows

a buffer system, but one of the ways to identify it is by determining if the solution

has a weak acid and its conjugate base. In this case, it is a buffer system, and this

equation is now available to us. First, we need to find the pKa using the Ka given in

the question stem. Then, we can plug in all of our known values to obtain the answer.

pKa = -log[Ka]

pKa = -log[1.8×10−5] ~ 4.7

After plugging in all of the known values from the question stem, it should look like

the following:

pH = 4.7 + log ( 0.1 / 0.1 ) = 4.7 + log (1) = 4.7 + 0 = 4.7

Notice how when the conjugate base and the acid have the same concentration,

the pH is equal to the pKa. This is a valuable property and idea to remember on test

day

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Day 11 MCAT Practice Question