Problem Set #1: Water, pH, and Buffers

1) Draw the hydrogen bonds that are likely to form between the following pairs of molecules:

b) two molecules of ethanol

c) two molecules of H₃C-NH-CO-CH₃

2) Identify the conjugate acids and bases in the following pairs of substances:

₃

⁺

₃

b) ⁺H₃N-CH₂-COO^-/⁺H₃N-CH₂-COOH

c) ⁺H₃N-CH₂-COO^-/H₂N-CH₂-COO^-

d)^-OOC-CH₂-COO^-/^-OOC-CH₂-COOH

3) Calculate the hydrogen ion concentration for each of the following substances:

b) Orange juice, pH 3.5

c) Urine, pH 6.2

d) Household ammonia, pH 11.5

e) Gastric juice, pH 1.8.

4) Calculate the pH of:

^-5

b) 3 x10^-6 N HCl

c) 10^-9 N HCl

d) 7 mM H₂SO₄

5) Suggest a suitable buffer range for each of the following substances:

b) Acetic acid (pK_a'=4.76) and its potassium salt.

6) What is the [CH₃-COO^-]/[CH₃-COOH] ratio in an acetate buffer at pH 5.00?

7) The pK_a' of ascorbic acid (vitamin c) is 4.2. At what pH is the ratio of the unprotonated to protonated forms of ascorbic acid:

(b) 1:10

(d) 1:3

8) For Ascorbic acid (pKa'= 4.2).

b) Which form predominates at physiological pH's?

c) Which form predominates at pH 12? At pH 1?

9) You need to carry out an enzymatic reaction at pH 7.5. A friend suggests a weak acid with a pK_a' of 3.9 as the basis of the buffer. Will this substance and its conjugate base make a suitable buffer? Why or why not?

10) How would you prepare one liter of 0.01 M phosphate buffer at pH 7.0, starting with a bottle of 0.01 M K₂HPO₄ and a bottle of 0.01 M KH₂PO₄? (The pKa' of phosphate buffer is 7.2.)

11) How would you prepare one liter of a 0.05 M phosphate buffer at pH 7.5 using crystalline K₂HPO₄ and a solution of 1 M HCl?

12) Oxalic acid (HOOC-COOH) has two dissociable protons with pKa's of 1.23 and 4.19.

b) What net charge does each ionic form have?

c) Draw a titration curve for oxalic acid

d) Label the titration curve with where each of the three ionic forms will predominate.

Solutions for Problem Set #1: