The titration curve for glycine is shown below. The key points are designated I to V.

Select the point on the titration curve where the average net charge of glycine is -1.

Question

The titration curve for glycine is shown below. The key points are designated I to V.

Select the point on the titration curve where the average net charge of glycine is -1.

BlackTom AI · Accepted Answer

To tackle this question, we first identify what the average net charge of glycine would be at different stages of the titration.
Option I: This point occurs at very low pH where both groups are protonated (NH3+ and COOH). The net charge here is close to +1, not -1, so this cannot be the point where the average charge is -1.
Option II: At around pH 2.34, the carboxyl group loses a proton giving -COO− while the amino group remains protonated (NH3+). The molecule carries a near-neutral or slightly positive charge overall, not -1, so this is not the correct point.
Option III: Near pH 5.97 (the isoelectric point for glycine), the average net charge is approximately zero because the positive and negative charges balance. Therefore, this point corresponds to net charge 0, not -1.
Option IV: Around pH 9.60, the amino group begins to deprotonate (NH3+ → NH2), while the carboxyl group remains deprotonated. This shifts the net charge toward negative values, and at this stage the average net charge crosses to -1 as the molecule loses the positive charge from the amino group while the carboxylate is already negative. Thus this point aligns with an average charge of -1.
Option V: At very high pH around 11.30, the amino group is fully deprotonated (NH2) and the carboxyl group is still deprotonated, giving a net charge of -1 or more negative depending on exact pH, but the titration curve shows the -1 region already established by IV. Since the question asks for where the average net charge is -1, the earlier, correct crossing occurs at IV, not necessarily at the extreme high pH point.
In summary, the charge transitions from +1 at very low pH to 0 near the isoelectric point, then toward negative values as the amino group deprotonates. The -1 state is achieved around the pH corresponding to point IV, making IV the correct choice.

The titration curve for glycine is shown below. The key points are designated I to V. Select the point on the titration curve where the average net charge of glycine is -1.单项选择题

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