For a buffer with pKa = 4.75, if the buffer's pH is 4.75, what is the ratio [A−]/[HA]?

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Multiple Choice

For a buffer with pKa = 4.75, if the buffer's pH is 4.75, what is the ratio [A−]/[HA]?

Explanation:
The Henderson–Hasselbalch relationship is what this question tests: pH equals pKa plus the log of the conjugate base to acid ratio, written as pH = pKa + log([A−]/[HA]). When the pH matches the pKa, the log term must be zero, which happens only if [A−]/[HA] = 1. So the concentrations of the conjugate base and the acid form are equal. This is the point where the buffer holds its pH most stably around that pKa. If the pH were one unit higher than the pKa, the ratio would be 10; if it were one unit lower, the ratio would be 0.1.

The Henderson–Hasselbalch relationship is what this question tests: pH equals pKa plus the log of the conjugate base to acid ratio, written as pH = pKa + log([A−]/[HA]). When the pH matches the pKa, the log term must be zero, which happens only if [A−]/[HA] = 1. So the concentrations of the conjugate base and the acid form are equal. This is the point where the buffer holds its pH most stably around that pKa. If the pH were one unit higher than the pKa, the ratio would be 10; if it were one unit lower, the ratio would be 0.1.

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