Can someone help explain how to do this problem?
- Thread starter Dhooy7
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Basically since it wants the conjugate base of the acid S you need to find the equivalence point (which is where the acids come together on the graph basically). Now for S it appears that it would be a PH of 6 because that's where the PH=Pka (the halfway point of acid S on the graph). However, It has to be more than 6 because it was titrated twice since the titrant used was .1M and the solution was .05M the solution had to be diluted twice to get .1M down to .05M. From this the only option that makes sense is C because it has to be greater than 6 so A and B don't work, and 12 is greater than all of the equivalent points.
I'm definitely not great at explaining things online, but I hope that helps in someway lol if you want a refresher look up a khan academy video on titration curves.
I'm definitely not great at explaining things online, but I hope that helps in someway lol if you want a refresher look up a khan academy video on titration curves.
This isn't the absolute right way to answer it but from a test taking strategy standpoint, I look at it like this:
Quick visual evaluation: Acid S buffers at pH 6, and when pure, appears to be pH 3.5-4.
Quick Concept 1: Conjugate bases are BASES, so we can immediately eliminate A. 4 and B. 6 because those are acidic pH. So we are left with C 9 and D 12.
Quick Concept 2: Weak acids generally have strong bases as their conjugate.
Quick Concept 3: Strong acids generally have weak bases as their conjugate.
Guessing rationale: If it's sitting at pH 4, I wouldn't call it super strong or super weak, so I'd just go with the more moderate C 9 rather than D 12 which is a very strong base.
This is one of those things where personally (as someone not aiming towards 520+), taking 10 seconds to generate a 50/50 guess is a much better use of your time vs spending a couple of minutes on the complexity of this problem. Taking 20-30 sec to generate a 60/40 or 70/30 guess is time management I can live with, especially in a timed situation like the MCAT.
If you actually get this concept, good. If you get this concept and can answer it within 60 seconds, THAT is what you're aiming for. If you get it, but it takes you 2-3 minutes to solve it, do a 15 second guess like I did, mark it for review later, and if you have time, come back and spend the 2-3 min to confirm.
Quick visual evaluation: Acid S buffers at pH 6, and when pure, appears to be pH 3.5-4.
Quick Concept 1: Conjugate bases are BASES, so we can immediately eliminate A. 4 and B. 6 because those are acidic pH. So we are left with C 9 and D 12.
Quick Concept 2: Weak acids generally have strong bases as their conjugate.
Quick Concept 3: Strong acids generally have weak bases as their conjugate.
Guessing rationale: If it's sitting at pH 4, I wouldn't call it super strong or super weak, so I'd just go with the more moderate C 9 rather than D 12 which is a very strong base.
This is one of those things where personally (as someone not aiming towards 520+), taking 10 seconds to generate a 50/50 guess is a much better use of your time vs spending a couple of minutes on the complexity of this problem. Taking 20-30 sec to generate a 60/40 or 70/30 guess is time management I can live with, especially in a timed situation like the MCAT.
If you actually get this concept, good. If you get this concept and can answer it within 60 seconds, THAT is what you're aiming for. If you get it, but it takes you 2-3 minutes to solve it, do a 15 second guess like I did, mark it for review later, and if you have time, come back and spend the 2-3 min to confirm.
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This is a read-the-graph question that should take you about twenty seconds to answer. First, the curve for Acid S is that of a weak acid, meaning that its pH at equivalence must be greater than 7 (because it has formed its conjugate base). This allows us to eliminate choices A and B. From here we can visually extrapolate the curve to its equivalence point and see that it is better described as 9 than 12. Pick choice C and move on.
If you happened to have studied using our books, then I hope you would have actually answered it the following way (in about ten seconds): Consider that at 25 mL you are at half equivalence, where pH = pKa. That appears to be about 6. The curve plateaus at about pH = 12 (when over titrated with excess base). The pH at equivalence is thus about 9, making choice C the best answer. In fact, had you used our method, I would have expected that you would have come up with pH = 9.35 in roughly ten seconds.
If you happened to have studied using our books, then I hope you would have actually answered it the following way (in about ten seconds): Consider that at 25 mL you are at half equivalence, where pH = pKa. That appears to be about 6. The curve plateaus at about pH = 12 (when over titrated with excess base). The pH at equivalence is thus about 9, making choice C the best answer. In fact, had you used our method, I would have expected that you would have come up with pH = 9.35 in roughly ten seconds.
