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 weak acid has a Ka of 1.8×10−5. If the initial concentration of HA is 0.1 M and it

partially dissociates in water, what is the equilibrium concentration of A?

A) [A] = 1.8 x 10-6 M

B) [A] = 1.8 x 10-4 M

C) [A] = 1.8 x 10-5 M

D) [A] = 1.8 x 10-3 M

Click to reveal answer

Correct answer: B. It’s important to keep in mind the equations

related to acid-base chemistry, remembering that Ka = [H+][A-] / [HA]. However,

since the degree of dissociation is relatively small, we can disregard the insignificant

change in HA concentration from the initial and final concentrations.

[HA] [H+] [A-]

Initial (M) 0.1 0 0

Change (M) ~0 +x +x

Equilibrium (M) 0.1 x x

As seen by this ICE table, the concentration of the final HA would not be very

different from the initial concentration. Therefore, the equilibrium [HA] = 0.1, as

stated in the question stem. There are still two unknowns, the [H+] and the [A-],

which we can call x. Now we have the following:

1.8×10−5 = x2 / 0.1

1.8×10−5 * 0.1 = x2

sqrt(1.8×10−6) = x ~ 1.8 x 10-4 M,

making B the correct answer choice. The remaining answers would be the result of

inaccurate calculations. Specifically, answer choice A would be if the last step of the

question has been skipped and the value of 1.8×10−6 was used.

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Day 9 MCAT Practice Question