How do we work the following problems? (Answers for 23 is C and the answer for 2
ID: 263054 • Letter: H
Question
How do we work the following problems? (Answers for 23 is C and the answer for 24 is A)
A. At what pH does a 0.02 M solution of histidine exhibit a net charge of 0.50? The pKa for the ????-carboxylate of histidine is 1.8 and the pKa for the ?-amino group of histidine is 9.2.
B. At what pH does a 0.02 M solution of alanine exhibit a net charge of 0.50? The pKa for the ????-carboxylate of alanine is 2.3 and the pKa for the ?-amino group of alanine is 9.7.
23. At what pH does a 0.02 M solution of histidine exhibit a net charge of 0.50? The pKa for the ?-carboxylate of histidine is 1.8 and the pKa for the a-amino group of histidine is 9.2. A. 1.8 B. 4.0 C. 6.0 D. 9.2 24. At what pH does a 0.02 M solution of alanine exhibit a net charge of 0.50? The pKa for the ?-carboxylate of alanine is 2.3 and the pKa for the ?-amino group of alanine is 9.7. A. 2.3 B. 4.0 C. 6.0 D. 9.7 E. 12.2Explanation / Answer
23. Amino acid having Acid-base properties. Histidine is a basic amino acid. It is Diamino monocarboxylic acid.
pK is the dissociation constant. pH=<pK1 (Cation), pH=>pK2 (Anion).
Normally amino acid in form of NH4+ and COOH form. Means it has a positive charge. Even presence of two amino groups, only one group having + charge. Here histidine, at pH 1.8 has a positive charge (1:COOH and 1:NH4+). At this pH carboxylic acid release H+ ion and gain -ve charge(COO-). PK2 is 9.2 where NH4+ converted to NH3. So < 9.2 pH and >1.8 pH the net charge of histidine is nearly 0. Standard amino acids don't have a buffering capacity at physiological pH. Only amino acid His having this property.In blood, His maintain H+ ion concentration. At pH 6.0, the reaction of COOH =COO- equilibrium state. By the addition of NaOH equalents, the pH changed by1/2.
24. Same as above. Ala is mono amino monocarboxylic acid (COOH and NH4+ form). Normally amino acid in form of NH4+ and COOH form. Means it has a positive charge. Here Ala, at pH 2.3 has a positive charge (1:COOH and 1:NH4+). At this pH carboxylic acid release H+ ion and gain -ve charge(COO-). PK2 is 9.7 where NH4+ converted to NH3. So < 9.7 pH and >2.3 pH the net charge of Ala is nearly 0/ 0.5. This is also equilibrium state at particular pH. So pH varies by 1/2 by adding NaOH equalents.
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