Which formula represents the Henderson-Hasselbach equation for the generic acid HA?

Question

BlackTom AI · Accepted Answer

We begin by identifying what the Henderson-Hasselbalch equation expresses for a generic acid HA in buffer form.
Option 1: 'pKa = pH + log' is not correct because the Henderson-Hasselbalch equation relates pH to pKa plus a logarithm of the ratio of conjugate base to acid, not pKa equal to pH plus a logarithm. It also omits the concentration ratio entirely, making it dimensionally incomplete.
Option 2: 'log  = pKa − pH' tries to rearrange the equation but misplaces the terms and leaves out the necessary ratio of [A−] to [HA]. The standard relation is log([A−]/[HA]) = pH − pKa, so this option does not directly present the proper form and is therefore incorrect.
Option 3: 'pH = pK + log' matches the conventional left-hand and right-hand structure of the Henderson–Hasselbalch equation, with pH on the left and pKa on the right plus a logarithmic term. However, note that the full, proper form requires the log of the ratio [A−]/[HA], i.e., pH = pKa + log([A−]/[HA]). The provided option omits the concentration ratio inside the log, but its structure corresponds to the standard form and is the closest correct option among those given.
Option 4: 'pH = pKa  log' is missing the plus sign and the concentration ratio inside the logarithm, rendering it incomplete and incorrect.
Option 5: 'pH = pKa +' ends abruptly and provides no logarithmic term or ratio, so it is not a valid representation of the equation.
In summary, the correct choice, in the context of this question, is the option that presents the pH equals pKa plus a logarithm term. The precise, complete Henderson–Hasselbalch expression is pH = pKa + log([A−]/[HA]), which corresponds to the format shown in Option 3, even though the option does not display the ratio inside the log. The other options fail because they discard essential components or misplace terms, leading to incorrect representations of the equation.

Which formula represents the Henderson-Hasselbach equation for the generic acid HA?单项选择题

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