Question

The graph below represents the titration of an amino acid with NaOH solution. View the titration and answer the questions below (parts a through e) 2 (a) At what pH is the average net charge -12? 2 Number --PKz 9.6 2.00 pl-6.01 pH 8- (b) Where does the amino acid have a net charge of +1? O below pH 2.34 O at pH- 2.34 O at pH 6.01 O at pH 9.69 O above pH 9.69 pKi 2.34 1.0 1.5 2.0 OH (oquivalents) (c) At what point has enough base been added to react with 2 of the NH3' groups? continued

Answer 1

(a). Half of the amine groups are ionized at pH=pKa of amine pKa of amine = 9.69 so half of the amino group is ionized at pH = 9.69 (6) Amino acid has net charge -1 when the dissociable protons are completely titrated with base. So, it is above pH=9.69 (c). Half of ammonium ion is reacted at the equilibrium point where there is equal concentration of protonated and deprotonated amine groups So, pH=9.69 Equivalence of OH-= 1.5equivalence of OH- (d). The best buffering capacity of the amino acid is at pH = 9.69 (e). The isoelectric point is the average of the two pKa values pI = mid-point on the graph pI = 6.01