Problem Set

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GENERAL BIOCHEMISTRY
Problem Set No. 1
Answered by: Andrea Rose A. Fajardo of BSFT 2-1N

1. Calculate the pH of a dilute solution that contains a molar ratio of potassium acetate to acetic acid (pKa=4.76) of: a. 2:1
pH = 4.76 + log[2]/[1]
= 4.76 + 0.3010
= 5.06
b. 1:4
pH = 4.76 + log[1]/[4]
= 4.76 + (-0.6021)
= 4.16
c. 6:5
pH = 4.76 + log[6]/[5]
= 4.76 + 0.0792
= 4.81
d. 2:3
pH = 4.76 + log[2]/[3]
= 4.76 + (-0.1761)
= 4.58
e. 9:1
pH = 4.76 + log[9]/[1]
= 4.76 + 0.9542
= 5.71
2. A buffer contains 0.01mol of lactic acid (pKa=3.86) and 0.05mol of sodium lactate per liter. a. What is the resulting pH of the buffer?
pH = 3.86 + log[0.05]/[0.01]
= 3.86 + 0.6990
= 4.56
b. Calculate the change in pH when 5mL of 0.5M HCl is added to 1L of buffer. 5mL of 0.5 mol/L
5mL * 1L/1000mL = 0.005L
0.005L * 0.5 mol/L = 0.0025mol
NaC3H5O3 = 0.05mol - 0.0025mol = 0.0475
C3H5O3 = 0.01mol - 0.0025mol =0.0075
pH = 3.86 + log[0.0475]/[0.0075]
= 3.86 + 0.8016
= 4.66
• 4.55-4.66 = 0.11 is the change in pH
c. What pH change would you expect if you added the same quantity of HCl to 1L of pure water? [H+] = (0.0025mol)(1L) = 0.0025M
pH = -log 0.0025
= 2.60
3. The structure of the amino acid of isoleucine is given at the right. a. How many chiral centers does it have? 2
b. How many optic isomers? 4
c. Draw the perspective formulas for all the optical isomers of isoleucine.

[pic]

4. A peptide has the sequence:
Glu-His-Trp-Ser-Leu-Arg-Pro-Gly
a. What is the charge of the molecules at pH 4 and 11? (Use pKa values of those in the ppt) |pH...
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