Question

2. Histidine is an amino acid important for tissue growth and repair. Histidine is metabolized by the body to produce histamine which is released in the body in response to trauma or to an allergic reaction, causing itchy eyes, stuffy sinuses, and general discomfort - sort of like exams. It also could potentially be a chelating agent. Let's give histidine the abbreviation Hz Hist. a. Given the 3 Kg 's for histidine, write the equilibrium reaction associated with each of the Ka's. Ka1 = 2.1 x 10-2 K 2 = 9.5 x 10-7 K 3 = 8.3 x 10-10 b. Calculate the pH of a 1.250 M Naz Hist solution. c. Calculate the pH of a buffer composed of 45.00 mLs of 1.250 M Na HHist and 55.00 mLs of 1.045 M NaH2Hist. d. Calculate the pH of this buffer (that in part c) if 16.90 mLs of 0.125 M NaOH are added to it.

Answer 1

A tri protic weak Acid H₃ Hist ką, - 2.1x162 kq, = 9.5x167 pkg, = -log(2-x 102) Pkq = -log (9.5x177) ulo - pka, = 1.678 PKQ2 = 6.022 kq2 = 8.3 x 10". pkog --loy 183 x17") Pkaz = 9.081 Vio a) Hz Histenst H2Oce) = H2 History + H30 torsika, Hz Hist (aq.) + H2O us - H Hist? (04) + H20 (164); Kaz H Hist?cm+ H2010) = Hist?cm) + Hjor comez skaz (Nag Hist) = 1.250 M pH = { (14 + pk az + logc) -{(14+ 9-08 1+log125) PH = 13-09 sme 6) - 11 we

(Na2H Hist] = 1.25M, usme nNagh Hist = (1+25x45) = 56.25 mmol ( NGH₂ Hist] = 1.045M, 5sme Naha Mist = (1.845x55) - 57.475 mmol pH = pka, + logo (Naz H Hist] J [na H₂ Hist = 6022 + longer [PH = 6:012 Ane d) 16.90 me of 0.125M NaOH is added in this buffer in na Olt - 16.90 0.125 = n NaOH = 2.1125 mmol NAH2 Hist + NaOH - NU, H Hist i nitical 57.475 2.1125 56.25 cm mol ) 58.3625 55: 3625 fineel Acidic buffer pH = pka + loe (Na2H Hist] (Na M₂ Hist = 6.022 + log (58.3625) (55.3625 [PH = 6.04 Ane