Le Chatelier's Principle: Why is my reasoning incorrect?

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Joined: Sun Dec 29, 2019 5:47 pm

Le Chatelier's Principle: Why is my reasoning incorrect?

Post by lotus0618 » Thu Jul 02, 2020 1:49 pm

"Which Physiological mechanism is Least likely to result in a significant decrease in blood acidity?

A. Exhalation of CO2

-my thought process: wrong cause less CO2 (stimulus) would make the reaction shift to the left to restore the equilibrium (response) -> higher pH instead

B. Release of H3PO4 into the bloodstream (correct answer)

--my thought process: H3PO4 in water would result in more H+ (stimulus), which shifts the reaction to the left to restore the equilibrium (response) -> higher pH.

But the explanation says that more H means more acidic. That's it. This doesn't make sense to me because, in reality, the reaction doesn't just stop there. Le Chatelier's principle states that the equilibrium will be corrected. Since my reasoning for A is correct, wouldn't this same reasoning be applied to B as well?

C. Conversion of Bicarbonate to CO2 in the blood

-More CO2 shifts the reaction to the right and thus makes it more acidic. So I chose this one as my answer.

But the explanation says that More CO2 means higher pH. Similar to B, I'm not convinced why the reaction just stops there. My reasoning for A works, so I don't see why my logic applied to both B and C are wrong. When there's a stimulus that disrupts the equilibrium, the reaction will shift to the left or right to correct the disruption. Therefore, when there's more CO2 in the blood, the reaction has to shift to the right to decrease the pH.

D. Excretion of protons into the urine by the kidneys

-fewer protons shift the reaction to the right so lower pH is maintained, so I was debating between C and D.

But the explanation says it's wrong because fewer protons mean less acidity.

Basically, I have 2 confusions:
1) why is my logic reasoning to A correct but this same logic doesn't work for the other options?
2) based on Le Chat. principle, when a stimulus is added and disrupts the equilibrium, the reaction will be shifted to recorrect the situation. I know that AAMC wants me to think only one layer deep without going further. However, why does my reasoning for A work then?

3) How would the question be phrased for my reasonings to be correct?
Posts: 57
Joined: Fri May 29, 2020 11:43 pm

Re: Le Chatelier's Principle: Why is my reasoning incorrect?

Post by NS_Tutor_Yuqi » Sat Jul 04, 2020 4:30 pm

Hi! I think the main thing that will help clear this up for you is remembering that pH is a direct measure of H+ concentration. Therefore, answer choices that directly impact H+ will have an immediate effect on pH, while choices that impact other components in the buffer system will have to go through Le Chat's principle in order to change H+ and pH levels.
A. Your reasoning is correct. Here, you had to use Le Chat in order to determine how H+ would be affected by changing CO2 levels.
B. You were correct in seeing how adding H3PO4 would increase H+ levels in the system. Although this will be balanced out by the buffer system, it has the immediate effect of lowering blood pH. Even when the buffer system kicks in and pH increases again, there will not be a situation where the pH would be higher than what it was before H3PO4 was added. If only a small amount of H3PO4 was added, then pH would be the same. However, if it was a large amount, then there might actually be a slight decrease in pH, even with the buffer system activated. Therefore, it has the potential of lowering blood pH, or is the least likely to decrease blood acidity.
C. Converting bicarb to CO2 is the effect of equilibrium shifting toward the left . This will lead to a decrease in H+, and therefore an increase in pH. Although I see your reasoning about high CO2 levels shifting the equation to the right, keep in mind that there was a physiological reason for the equation shifting left in the first place. Therefore, it wouldn't make sense for the body to just shift it back again.
D. Since pH directly correlates to H+ levels, removing H+ through urine will increase pH levels. This was probably the result of having too much H+ in the first place, which is why the kidneys would remove them. Therefore, this process is what allows the body to shift back to equilibrium.

when encountering problems like this in the future, just remember that AAMC most likely won't ask you to go through multiple layers of thinking to get to the answer. Another thing to notice is that B is adding H+ and D is taking away H+. Since we're concerned with pH, odds are that one of these will be the right answer since they oppose each other. This isn't a hard set rule, but a good rule of thumb for all MCAT questions. I hope this helps clarify things!
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