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).

HCl (0.1 M) and acetic acid (0.1 M) are put in two different beakers at the same

concentration. Which statement best describes their dissociation and pH

levels?

A) HCl completely dissociates and has a higher pH compared to CH3

COOH

B) HCl completely dissociates and has a lower pH compared to CH3

COOH

C) CH3

COOH completely dissociates and has a lower pH compared to HCl

D) CH3

COOH partially dissociates and has the same pH

Click to reveal answer

Correct answer: B. Since HCl is one of the strong acids, it will

dissociate completely. Acetic acid is not one of the strong acids and, therefore,

will not dissociate completely. For these types of questions, it is imperative to

memorize the list of strong acids and bases and refer to all other acids or bases

as weak. In this case, since HCL is a strong acid and dissociates completely,

answer choices A and B are the best candidates so far, and answers C and D are

eliminated. When finding the pH, it is important to know what pH means. pH =

-log[H+]. Therefore, as H+ concentration increases, the pH decreases. This means

that whichever species produces the most free H+ concentration would have the

lowest pH. Since HCl and acetic acid have the same number of H+ to lose (they

are both monoprotic), the amount of dissociated H+ is determined by their acid

strength. The stronger the acid, the better it is at donating H+. In this case, HCl

is a strong acid, which means it dissociates completely into H+ ions, while acetic

acid only partially dissociates, creating a fraction of the H+ that HCl donates. Since

HCl solution creates more free H+, it naturally would have a lower pH, thus making

answer B correct over A.

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Day 10 MCAT Practice Question