Practice pH and Henderson Hasselbalch Problems

1. Using the Henderson-Hasselbalch equation, explain why when a solution is at a pH below the pKa for an acid that more than 50% of the molecules have the proton on (not off) - ANSWER = When pH<pKa, rearrangement of Henderson-Hasselbalch equation yields a negative number = log (salt)/acid). The log of a number is negative when the denominator is greater than the numerator.

2. A new amino acid is discovered, called ahernium. It contains a two carboxyls, and three amines. The pKas are

Carboxyl #1 pKa = 2.2
Carboxyl #2 pKa = 4.4
Amine#1 pKa = 5.0
Amine#2 pKa = 8.5
Amine#3 pKa = 9.5

Predict the charge on ahernium at the following pHs:

1.0 - ANSWER = +3
7.0 - ANSWER = 0
11 - ANSWER = -2

What do you predict would be the pI of ahernium? - ANSWER = 6.75

3. What is the pH of a solution of

a. 0.023 M HCl - ANSWER = 1.6

b. 0.18 M NaOH - - ANSWER = 13.26

c. a salt - acid mixture with twice the concentration of salt compared to acid (if the pKa is 4.1) - ANSWER = 4.4

4. What is the pKa of a salt-acid mixture if the solution has a pH of 3.3 when there is three times the concentration of acid as compared to salt? - ANSWER = 3.78

5. What is the hydrogen ion concentration of a solution with a pH of 4.3? - ANSWER = .00005 M (5 x 10-5 M)