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Choose the correct option for structures of A and B, respectively CH(CH3)2 | H2N — CH — COOH pH = 2 → A pH = 10 → B

1. H2N — CH — COO− | CH(CH3)2 and H3N+ — CH — COOH | CH(CH3)2
2. H2N — CH — COO− | CH(CH3)2 and H3N+ — CH — COO− | CH(CH3)2
3. H3N+ — CH — COO− | CH(CH3)2 and H3N+ — CH — COOH | CH(CH3)2
4. H3N+ — CH — COOH | CH(CH3)2 and H2N — CH — COO− | CH(CH3)2

Correct answer: H3N+ — CH — COOH | CH(CH3)2 and H2N — CH — COO− | CH(CH3)2

Solution

At pH 2, the amino group is protonated (H3N+) and the carboxylic acid group is still in its acidic form (COOH), while at pH 10, the carboxylic acid group loses a proton to become deprotonated (COO−), resulting in the correct structures for A and B.

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