Bb 450 Assignment

Cody Mead

Henderson-Hasselbalch Practice Problems for BB 450



Useful pKas - Acetic acid = 4.76. For the phosphate buffers below, you can use pKa1 = 2.1, pKa2 = 6.83, and pKa3 = 10.1


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).

H-H: pH = pKa + Log [S]/[A]

If [A] is larger than [S] (or that there are more protons on than off) then the ratio is less than 1, and when you take the log of a number less than 1, the result is negative.

Then (pH = pKa + some negative number) or that pH is less than the pKa.



2. Using the Henderson-Hasselbalch equation, determine the ratio of acetate ion (salt) to acetic acid (acid) present in a 1 M solution at the following pHs


H-H: pH = pKa + Log [S]/[A]

a. pH = 1: 1 = 4.76 + Log [S]/[A] -> -3.76 = Log [S]/[A] -> 10^-3.76 = ratio

b. pH = 2: 1 = 4.76 + Log [S]/[A] -> -2.76 = Log [S]/[A] -> 10^-2.76 = ratio

c. pH = 3: 1 = 4.76 + Log [S]/[A] -> -1.76 = Log [S]/[A] -> 10^-1.76 = ratio

d. pH = 5: 1 = 4.76 + Log [S]/[A] -> 0.24 = Log [S]/[A] -> 10^0.24 = ratio

e. pH = 7: 1 = 4.76 + Log [S]/[A] -> 2.24 = Log [S]/[A] -> 10^2.24 = ratio

f. pH = 9: 1 = 4.76 + Log [S]/[A] -> 4.24 = Log [S]/[A] -> 10^4.24 = ratio



3. At physiological pH (7.0), what will be the predominant form of phosphate present in cells? (H3PO4? H2PO4-? HPO4-2, or PO4-3?) Justify your answer using the Henderson-Hasselbalch equation.

H-H: pH = pKa + Log [S]/[A], pKa1 = 2.1, pKa2 = 6.83, pKa3 = 10.1

At pKa2 (6.83) the value is closest to the desired pH (7.0), so the most important portion to look at is going to be the [HPO4-2]/[ H2PO4-] ratio.

pH = 7.0, pKa = 6.83

7.0 = 6.83 + Log [HPO4-2]/[ H2PO4-] -> 0.17 = Log [HPO4-2]/[ H2PO4-]

Since the log of the ratio results in a positive number, there must be more Salt than Acid, so the predominant form of phosphate would be HPO4-2

4. Formic...