Question

The pKa of the α-carboxyl group of phenylalanine is 1.83, and the pKa of its α-amino group is 9.13. Calculate the average net charge on phenylalanine if it is in a solution that has a pH of 8.20. If the charge is positive, do not enter a + sign.

Answer 1

Using Henderson Hasselbalch equation

find ratio of deprotonated and protonated form can be calculated as follows:

pH = pKa + log [COO-]/[COOH]

substitute data

8.5 = 1.83 + log [COO-]/[COOH]

log [COO-]/[COOH] = 8.5-1.83

log [COO-]/[COOH] 6.67

[COO-]/[COOH] = 10^6.677

Thus, deprotonated form will be in large excess.

[COO-]:[COOH]= 10^6.677: 1

Thus

charge due to deprotonated carboxylic acid can be calcualted as follows:

charge due to deprotonated carboxylic acid = -(10^6.677)/(10^6.677+1)

charge due to deprotonated carboxylic acid = -0.99

charge due to deprotonated carboxylic acid = -0.99

The pKs of amino group is 9.13.

The pH of phenyl alanine solution is 8.50.

Since, pKa is more than pH, amino group will be protonated and it will have positive charge.

Using Henderson Hasselbalch equation

Find againt the ratio of deprotonated and protonated form can be calculated as follows:

pH = pKa + log [NH2]/[NH3+]

8.50 = 9.13 + log [NH2]/[NH3+]

log [NH2]/[NH3+] = 8.50 – 9.13

log [NH2]/[NH3+] = -0.63

invert

log [NH3+]/[NH2] = 0.63

[NH3+]/[NH2] = 10^ 0.63= 4.26

Thus,

[NH3+]:[NH2] = 4.26 : 1

Therefore, charge on protonated amino group can be calculated as follows:

Charge due to protonated amino group = +4.26 /(4.26+1)

Charge due to protonated amino group= +0.809

Thus, net charge on phenyl alanine can be calculated as follows:

Net charge = charge on deprotonated carboxylic acid + charge on protonated amino group

Net charge = -0.99 + 0.809

Net charge = -0.181

AVG charge -0.18 approx