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).

If the pH is 2.3, what is the concentration of hydroxide in the solution?

A) 10 11.7 M

B) 10 -2.3 M

C) 10 -11.7 M

D) 10 2.3 M

Click to reveal answer

Correct answer: C. First, it is important to understand the question

stem, which asks for the [OH] in the solution, which is the concentration of OH-.

Since pH + pOH = 14 and pH is 2.3, this means that pOH = 14 - pH = 14 - 2.3. Thus,

pOH is 11.7. Since pOH = -log[OH], we must reverse this equation, first algebraically

manipulating this equation to [OH] = 10 - pOH; [OH] = 10 -11.7. Therefore, answer

choice C is correct. Answer choice A only makes sense if the negative sign has been

ignored or forgotten. Since the given value was 2.3, it might have been tempting to

find the concentration of H with that number and derive either answers B or D, but

these don’t give the concentrations of OH.

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Day 8 MCAT Practice Question