Question

Given a 0.2 M histidine solution, what is the concentration of the acid form (HA) and its conjugate base (A^-) at the isoelectric point?

a) 0.2 M and 0.2 M. b) 0.1 M and 0.1 M. c) 0.2 M and 0.4 M. d) 0.4 M and 0.4 M

Answer 1

At the isoelectric point of histidine, the concentrations of the acid form (HA) and the conjugate base (A-) are equal. Given the total solution concentration of 0.2 M, each concentration would be 0.1 M, which corresponds to option b) 0.1 M and 0.1 M.

Step-by-step explanation:

The question asks for the concentration of the acid form (HA) and its conjugate base (A-) of histidine at the isoelectric point. At a molecule's isoelectric point (pI), the concentration of its acidic form and its conjugate base are equal because the molecule has no net charge, meaning it's at a pH where it doesn't tend to donate or accept protons.

Given that the total concentration of the histidine solution is 0.2 M and assuming that histidine only has one acidic proton that significantly contributes to its isoelectric point, at the isoelectric point, the histidine exists as half HA and half A-. Therefore, the concentration of HA and A- both would be 0.1 M, because the original histidine concentration (0.2 M) is divided equally between HA and A- at the isoelectric point.

The correct answer is therefore: b) 0.1 M and 0.1 M.