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Janelle_5719
#1 Posted : Tuesday, June 02, 2020 10:31:12 PM
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Hi there,

For #163 on Pg.253 of the chemistry study guide, how do you choose between choices A and B because both would have a range that seems to suit the PH?
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INSTR_Kailey_109
#2 Posted : Friday, June 05, 2020 6:59:06 PM
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It’s true that both are good answers (you’ve eliminated to a 50/50 chance which is great), but the MCAT is about finding the best answer. In this case, the best answer is the answer with the pKa that is closest to the expected equivalent point (ie pH 8.2). To compare A and B, we calculate that the pKa is 8 for phenolphthalein and approximately 7 for bromothymol blue (~1x10^-7). Therefore, A is the best answer.
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