Possible error in TBR Chem answer choice?

[deedave33]

---

Members don't see this ad.

For question 92 on page 336 of chem book 1: Isn't answer choice D also correct?

Question is:

92. An aqueous solution that has a hydroxide concentration
of 1.0 x 10-6 M would show what colors?
A. Yellow with bromcresol green
B. Red with methyl red
C. Blue with bromthymol blue
D. Clear with phenolphthalein

TBR Answer: C

 

[BerkReviewTeach]

For question 92 on page 336 of chem book 1: Isn't answer choice D also correct?

Question is:

92. An aqueous solution that has a hydroxide concentration
of 1.0 x 10-6 M would show what colors?
A. Yellow with bromcresol green
B. Red with methyl red
C. Blue with bromthymol blue
D. Clear with phenolphthalein

TBR Answer: C

If hydroxide concentration is 10-6 M, then the pOH is 6, making the pH = 8. Phenolphthalein has a pKa of 8.79, so the indicator will be about 7 parts clear (protonated form) and 1 part magenta (deprotonated form). A solution that has one out of eight indicator molecules in the magenta state would likely have a light pink color. So choice D is in the grey area of what the question is looking for.

Bromthymol blue has a pKa of 6.78, so at pH = 8.0, the indicator will be about 1 part yellow (protonated form) and 15 parts blue (deprotonated form), resulting in a blue solution. Choice C is a true statement.

The tricky thing about some MCAT questions is that you are looking for the best answer, and not necessarily a perfect answer. That's the case here. Choice C is correct and choice D is kind of right. The better choice is C, because when the pH is more than one unit from the pKa, then the color is outside of the mixed color range. This varies with color change and intensity, but it's a good general rule.

 

[MLT2MT2DO]

I would say I have had close to a dozen times where I've swore TBR had the incorrect answer, I think out of those 12 times one turned out to be correct.

TBR is pretty spot on, especially with their chem.

 

[BerkReviewTeach]

I would say I have had close to a dozen times where I've swore TBR had the incorrect answer, I think out of those 12 times one turned out to be correct.

TBR is pretty spot on, especially with their chem.

Each chapter has about 5% of the questions with some ambiguity (such as two correct answers where one is better) or you're forced to choose a best of four imperfect answers. They do this on purpose to prepare for the wth questions that show up on the MCAT from time to time. It can be frustrating dealing with these questions, but in the end, you learn a lot from them.