Question 19 of 26 Incorrect Map Sapling Learning The structure of tryptophan is

Question

Question 19 of 26 Incorrect Map Sapling Learning The structure of tryptophan is shown at the right along with the pK values of the a-carboxyl and pK2 9.39 H3N CH-C OH pK 2.38 -amino groups. Tryptophan is a diprotic acid which can exist in CH three ionized forms: H2A HAO, and A (HA consists primarily of the zwitterionic form with a negatively charged c group and positively charged amino group, but a very small amount of the completely neutral species does exist). HN Calculate the fraction (a) of each species present at pH 10.1 Number Number Number 7.74 x 10 11 a 0.00405 at 0.02079 HA" H.A.

Explanation / Answer

Given pK1 and pK2, find K1 and K2 as

K1 = 10^(-pK1) = 0.0041687

K2 = 10^(-pK2) = 4.0738*10-10

Also, pH of the solution is 10.1; therefore,

[H+] = 10^(-pH) = 10^(-10.1) = 7.94328*10-11

Use the empirical formula and plug in values to obtain

(H2A+) = [H+]2/[H+]2 + K1[H+] + K1K2 = (7.94328*10-11)2/[(7.94328*10-11)2 + (0.0041687).(7.94328*10-11) + (0.0041687).(4.0738*10-10)] = (5.61492*10-21)/[(5.61492*10-21) + (3.311315*10-13) + (1.69824*10-12)] = (5.61492*10-21)/(2.02937*10-12) = 2.7668*10-9 2.77*10-9 (ans).

(HA0) = K1[H+]/[H+]2 + K1[H+] + K1K2 = (0.0041687).(7.94328*10-11)/(2.02937*10-12) = 0.1631 0.163 (ans).

(A-) = K1K2/[H+]2 + K1[H+] + K1K2 = (0.0041687).(4.0738*10-10)/(2.02937*10-12) = 0.8368 0.837 (ans)

The sum will be slightly more than 1 and this is due to rounding off of digits (ans).

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