Question

13. Imagine there is one large molecule with 4 carboxylic acids. It has 4 pKas: 2, 4, 6, 8. What is the overall net charge of the molecule in a solution with a pH of 1? A. O B. -1 C. +1 D. -2 E. + 2 F. -3 G. +3 H.-4 14. For the same molecule in the above question, what is its overall net charge in a neutral solution? A. O B. -1 1 C. + D . -2 E. +2 G. +3 H.-4 1. +4 F. -3 D. H.-4 15. What is its overall net charge of the same molecule in a solution with a pH of 10? A. O B. -1 C. +1 E. +2 F.-3 G. +3 I. +4 22. Lactic acid (HA)/lactate (A) are produced as a byproduct of anaerobic respiration in skeletal muscle. Calculate the ratio of lactate:lactic acid in the blood stream (pH = 7.4) and the pka of lactic acid is 3.86. A. 1:225 B. 1:3,467 C. 225:1 D. 3,467:1

Answer 1

13.

Correct option: A. 0

At pH = pK_a, half of the acid is dissociated.

The pH value of 1 is below the pK_a of all four carboxylic acid groups. Hence, none of them is dissociated at this pH. Hence, the net charge of the molecule in the solution = 0

14.

Correct option: F. - 3

Neutral solution means pH = 7.

At pH = pK_a, half of the acid is dissociated.

Three of the four carboxylic acid groups have pK_a below 7. Hence, 3 carboxylic acid groups will be dissociated at pH 7. Hence, the net charge of the molecule in solution at pH 7 = - 3

15.

Correct option: H. - 4

At pH = pK_a, half of the acid is dissociated.

All four carboxylic acid groups have pK_a below 10. Hence, all 4 carboxylic acid groups will be dissociated at pH 10. Hence, the net charge of the molecule in solution at pH 10 = - 4

16.

Correct option: D. 3467 : 1

From Henderson-Hasselbalch equation,

pH = pK_a + log([lactate, A-]/[lactic acid, HA])

or, 7.4 = 3.86 + log([lactate, A-]/[lactic acid, HA])

or, log([lactate, A-]/[lactic acid, HA]) = 3.54

or, [lactate, A-]/[lactic acid, HA] = 10^3.54 = 3467