A histidine residue (B), one of the functional groups in the structure of a certain enzyme, has

Question:

A histidine residue (B), one of the functional groups in the structure of a certain enzyme, has a conjugate-acid pK_a = 7.8. What is the fraction of each form (BH and B) present at physiological pH (pH = 7.4)? (enzyme) :N CH H B N: + H0 (enzyme) CH :N- H + NH BH (pK = 7.8) + HO