Chp 2

56.What quality of zwitterions makes them desirable buffers?

Zwitterions tend not to interfere with biochemical reactions

25. Calculate the hydroxide ion concentration,[OH−], for each of the materials used in Question 24.

[OH]=3.2*10-8M [OH]=7.9*10-8M [OH] = 2.0 * 10-10 M [OH] = 1.6 * 10-11 M

38.Calculate the pH of a buffer solution prepared by mixing 25 mL of 1.0 M lactic acid and 75 mL of 1.0 M sodium lactate

check book

39. Calculate the pH of a buffer solution that con- tains 0.10 M acetic acid (Table 2.6) and 0.25 M sodium acetate.

check book

35.How would you prepare 1 L of a 0.050 M phosphate buffer at pH 7.5 using crystalline K2HPO4 and a solution of 1.0 M HCl?

check book At pH 7.5, the ratio of [HPO422]/[H2PO42] is 2/1 (pKa of H2PO425 7.2), as calculated using the Henderson-Hassel- balch equation. K2HPO4 is a source of the base form, and HCl must be added to convert one-third of it to the acid form, according to the 2/1 base/acid ratio. Weigh out 8.7 g of K2HPO4 (0.05 mol, based on a formula weight of 174 g/mol), dissolve it in a small quality of distilled water, add 16.7 mL of 1 M HCl (gives 1/3 of 0.05 mol of hydrogen ion, which converts 1/3 of the 0.05 mol of HPO422 to H2PO42), and dilute the resulting mixture to 1 L.

43.If you have 100 mL of a 0.10 M TRIS buffer at pH 8.3 (Table 2.8) and you add 3.0 mL of 1 M HCl, what will be the new pH?

check book First calculate the moles of buffer that you have: 100 mL 5 0.1 L, and 0.1 L of 0.1 M TRIS buffer is 0.01 mol. Since the buffer is at its pKa, there are equal concentrations of the acid and basic form, so the amount of TRIS is 0.005 mol, and the amount of TRIS-H1 is 0.005 mol. If you then add 3 mL of 1 M HCl, you will be adding 0.003 mol of H1. This reacts as shown: TRIS 1 H1 : TRIS-H1 until you run out of something, which will be the H1, since it is the limiting reagent. The new amounts can be calculated as shown: TRIS-H1 5 0.005 mol 1 0.003 mol 5 0.008 mol TRIS 5 0.005 mol 2 0.003 mol 5 0.002 mol Now plug these values into the Henderson-Hasselbalch pH = 7.70

31.What is the [CH3COO−]/[CH3COOH] ratio in an acetate buffer at pH 5.00?

check book Use the Henderson-Hasselbalch equation:(1/7)/1

(e) Equivalence point

e) The point in a titration curve at which the added acid or base equals the amount of buffer originally present

(h) Nonpolar

h) The property of a molecule that is not soluble in water. The property of a covalent bond in which there is even sharing of electrons and no dipole moments (partial charges).

(i) Polar

i)The property of a molecule that is soluble in water. The property of a covalent bond in which the electrons are not shared evenly and dipole moments (partial charges) exist

26.Define the following: (a) Acid dissociation constant

(a) The numerical constant equal to the concentration of the products of the dissociation divided by the concentration of the undissociated acid form: ([H1][A2])/[HA].

(b) Acid strength

(b) The qualitative or quantitative description of how much acid (HA) dissociates to hydrogen ion

(c) Amphipathic

(c) The property of a molecule that has both a polar region and a nonpolar region.

(d) Buffering capacity

(d) The amount of acid or base that can be added to a buffer before experiencing a sharp pH change

(f) Hydrophilic

(f) The property of a molecule that is readily soluble in water (i.e., water-loving).

(g) Hydrophobic

(g) The property of a molecule that is insoluble in water (i.e., water-hating)

(j) Titration

(j)An experiment in which acid or base is added stepwise to a solution of a compound and the pH is measured as a function of the added substance.

5.What is a salt bridge?

A salt bridge refers to electrostatic attractions of parts of molecules with others. An example would be a negatively charged side chain from an aspartate residue in a protein with a positively charged lysine residue.

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

A substance with a pKa of 3.9 has a buffer range of 2.9 to 4.9. It does not buffer effectively at pH 7.5.

37.Calculate the pH of a buffer solution prepared by mixing 75 mL of 1.0 M lactic acid (see Table 2.6) and 25 mL of 1.0 M sodium lactate.

After mixing, the buffer solution (100 mL) contains 0.75 M lactic acid and 0.25 M sodium lactate. The pKa of lactic acid is 3.86. Use the Henderson-Hasselb equation check book

21.Aspirin is an acid with a pKa of 3.5; its structure includes a carboxyl group. To be absorbed into the bloodstream, it must pass through the membrane lining the stomach and the small intestine. Electrically neutral molecules can pass through a membrane more easily than can charged molecules. Would you expect more aspirin to be absorbed in the stomach, where the pH of gastric juice is about 1, or in the small intestine, where the pH is about 6? Explain your answer.

Aspirin is electrically neutral at the pH of the stomach and can pass through the membrane more easily than in the small intestine.

23.Calculate the hydrogen ion concentration, [H1], for each of the following materials: (a) Blood plasma, pH 7.4 (b) Orange juice, pH 3.5 (c) Human urine, pH 6.2 (d) Household ammonia, pH 11.5 (e) Gastric juice, pH 1.8

Blood plasma, pH 7.4 [H1] = 4.0 * 10-8 M Orange juice, pH 3.5 [H ] = 3.2 * 10-4M M Human urine, pH 6.2 [H1] = 6.3 * 10-7 M household ammonia pH 1.8 [H]= 3.2 *10-12 M gastric juice pH 6.5 [H]= 1.6*10-2

51. The solution in Question 35 is called 0.050 M, even though the concentration of neither the free base nor the conjugate acid is 0.050 M. Why is 0.050 M the correct concentration to report?

Buffer concentrations are typically reported to be the sum of the two ionic forms.

53.book Define buffering capacity. How do the following buffers differ in buffering capacity? How do they differ in pH? Buffer a: 0.01 M Na2HPO4 and 0.01 M NaH2PO4 Buffer b: 0.10 M Na2HPO4 and 0.10 M NaH2PO4 Buffer c: 1.0 M Na2HPO4 and 1.0 M NaH2PO

Buffering capacity is based on the amounts of the acid and base forms present in the buffer solution. A solution with a high buffering capacity can react with a large amount of added acid or base without drastic changes in pH. A solution with a low buffering capacity can react with only comparatively small amounts of acid or base before showing changes in pH. The more concentrated the buffer, the higher is its buffering capacity. Buffer (a) has one-tenth the buffering capacity of buffer (b), which in turn has one-tenth the buffering capacity of buffer (c). All three buffers have the same pH, because they all have the same relative amounts of the acid and base form

15.What are the requirements for molecules to form hydrogen bonds? (What atoms must be present and involved in such bonds?)

For a bond to be called a hydrogen bond, it must have a hydrogen covalently bonded to O, N, or F. This hydrogen then forms a hydrogen bond with another O, N, or F.

17. check book How many water molecules could hydrogen- bond directly to the molecules of glucose, sorbitol, and ribitol, shown here?

Glucose 17, sorbitol 18, ribitol 15; each alcohol group can bond to three water molecules and the ring oxygen binds to two. The sugar alcohols bind more than the corresponding sugars.

24.Calculate the hydrogen ion concentration, [H1], for each of the following materials: (a) Saliva, pH 6.5 (b) Intracellular fluid of liver, pH 6.9 (c) Tomato juice, pH 4.3 (d) Grapefruit juice, pH 3.2 check book

Saliva, pH 6.5 [H]=3.2*10-7M Intracellular fluid (liver), pH 6.9 [H1]=1.6*10-7M Tomato juice, pH 4.3 [H]=5.0*10-5M Grapefruit juice, pH 3.2 [H]=6.3*10-4M

59. book Identify the zwitterions in the list of sub- stances in Question 19.

The only zwitterion is 1H3N!CH2!COO2.

29.List the criteria used to select a buffer for a biochemical reaction.

The pK of the buffer should be close to the desired buffer pH, and the substance chosen should not interfere with the reaction being studied.

30.What is the relationship between pKa and the useful range of a buffer?

The useful pH range of a buffer is one pH unit above and below its pKa.

33.What is the ratio of TRIS/TRIS-H1 in a TRIS buffer at pH 8.7

Use the Henderson-Hasselbalch eSquation: check book