MCAT Biology Question — Predominate Structures
- Apr 04, 2017
- MCAT Question of the Day
Click for Explanation
pH = pKa + log[COO–]/[COOH]
7.4 = 2.34 + log[COO–]/[COOH]
5.06 = log[COO–]/[COOH]
105.06 = [COO–]/[COOH]
[COO–] = 105.06[COOH]
Thus, the conjugate base COO= is the predominate form.
pH = pKa + log[NH2]/[NH3+]
7.4 = 9.60 + log[NH2]/[NH3+]
-2.2 = log[NH2]/[NH3+]
10-2.2 = [NH2]/[NH3+]
102.2[NH2] = [NH3+]
Thus, the acid NH3+ predominates.
Therefore, the structure in answer choice D is the most common form of glycine at a physiological pH.
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