Chapter #2 : Water Chapter Problems and Answers Summary (Lecture 11)1. Effect of Local Environment on Ionic Bond Strength If the ATP-binding site of an enzyme is buried in the interior of the enzyme, in a hydrophobic environment, is the ionic interaction between enzyme and substrate stronger or weaker than that same interaction would be on the surface of the enzyme, exposed to water? Why? 2. Biological Advantage of Weak Interactions The interactions between biomolecules are often stabilized by weak interactions such as hydrogen bonds. How might this be an advantage to the organism? 3. Solubility of Ethanol in Water Explain why ethanol (CH3CH2OH) is more soluble in water than is ethane (CH3CH3). 4. Calculation of pH from Hydrogen Ion Concentration What is the pH of a solution that has an H+ concentration of (a) 1.75 × 10–5 mol/L; (b) 6.50 × 10–10 mol/L; (c) 1.0 × 10–4 mol/L; (d) 1.50 × 10–5 mol/L? 5. Calculation of Hydrogen Ion Concentration from pH What is the H+ concentration of a solution with pH of (a) 3.82; (b) 6.52; (c) 11.11? 6. Acidity of Gastric HCl In a hospital laboratory, a 10.0 mL sample of gastric juice, obtained several hours after a meal, was titrated with 0.1 M NaOH to neutrality; 7.2 mL of NaOH was required. The patient’s stomach contained no ingested food or drink; thus assume that no buffers were present. What was the pH of the gastric juice? 7. Calculation of the pH of a Strong Acid or Base (a) Write out the acid dissociation reaction for hydrochloric acid. (b) Calculate the pH of a solution of 5.0 × 10–4 M HCl. (c) Write out the acid dissociation reaction for sodium hydroxide. (d) Calculate the pH of a solution of 7.0 × 10–5 M NaOH. 8. Calculation of pH from Concentration of Strong Acid Calculate the pH of a solution prepared by diluting 3.0 mL of 2.5 M HCl to a final volume of 100 mL with H2O. 9. Measurement of Acetylcholine Levels by pH Changes The concentration of acetylcholine (a neurotransmitter) in a sample can be determined from the pH changes that accompany its hydrolysis. When the sample is incubated with the enzyme acetylcholinesterase, acetylcholine is converted to choline and acetic acid, which dissociatesto yield acetate and a hydrogen ion: In a typical analysis, 15 mL of an aqueous solution containing an unknown amount of acetylcholine had a pH of 7.65. When incubated with acetylcholinesterase, the pH of the solution decreased to 6.87. Assuming there was no buffer in the assay mixture, determine the number of moles of acetylcholine in the 15 mL sample.10. Physical Meaning of pKa Which of the following aqueous solutions has the lowest pH: 0.1 M HCl; 0.1 M acetic acid (pKa = 4.86); 0.1 M formic acid (pKa = 3.75)? 11. Meanings of Ka and pKa (a) Does a strong acid have a greater or lesser tendency to lose its proton than a weak acid? (b) Does the strong acid have a higher or lower Ka than theweak acid? (c) Does the strong acid have a higher or lower pKa than the weak acid? 12. Simulated Vinegar One way to make vinegar (not the preferred way) is to prepare a solution of acetic acid, the sole acid component of vinegar, at the proper pH (see Fig. 2-15) and add appropriate flavoring agents. Acetic acid (Mr 60) is a liquid at 25 °C, with a density of 1.049 g/mL. Calculate the volume that must be added to distilled water to make 1L of simulated vinegar (see Fig. 2-16). 13. Identifying the Conjugate Base Which is the conjugate base in each of the pairs below? (a) RCOOH, RCOO− (b) H2PO4- , H3PO4 (c ) RNH2 , RNH3+ (d) H2CO3 , HCO-314. Calculation of the pH of a Mixture of a Weak Acid and Its Conjugate Base 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; (b) 1:3; (c) 5:1; (d) 1:1; (e) 1:10. 15. Effect of pH on Solubility The strongly polar, hydrogen-bonding properties of water make it an excellent solvent for ionic (charged) species. By contrast, nonionized, nonpolar organic molecules, such as benzene, are relatively insoluble in water. In principle, the aqueous solubility of any organic acid or base can be increased by converting the molecules to charged species. For example, the solubility of benzoic acid in water is low. Theaddition of sodium bicarbonate to a mixture of water and benzoic acid raises the pH and deprotonates the benzoic acid to form benzoate ion, which is quite soluble in water. Are the following compounds more soluble in an aqueous solution of 0.1 M NaOH or 0.1 M HCl? (The dissociable protons are shown in red.) 16. Treatment of Poison Ivy Rash The components of poison ivy and poison oak that produce the characteristic itchy rash are catechols substituted with long-chain alkyl groups. If you were exposed to poison ivy, which of the treatments below would you apply to the affected area? Justify your choice.(a) Wash the area with cold water.(b) Wash the area with dilute vinegar or lemon juice.(c) Wash the area with soap and water.(d) Wash the area with soap, water, and baking soda (sodium bicarbonate). 17. pH and Drug Absorption Aspirin is a weak acid with a pKa of 3.5 (the ionizable H is shown in red): It is absorbed into the blood through the cells lining the stomach and the small intestine. Absorption requires passage through the plasma membrane, the rate of which is determinedby the polarity of the molecule: charged and highly polar molecules pass slowly, whereas neutral hydrophobic ones pass rapidly. The pH of the stomach contents is about 1.5, and the pH of the contents of the small intestine is about 6. Is more aspirin absorbed into the bloodstream from the stomach or from the small intestine? Clearly justify your choice. 18. Calculation of pH from Molar Concentrations What is the pH of a solution containing 0.12 mol/L of NH4Cl and 0.03 mol/L of NaOH (pKa of pKa of is 9.25)? 19. Calculation of pH after Titration of Weak Acid A compound has a pKa of 7.4. To 100mL of a 1.0 M solution of this compound at pH 8.0 is added 30 mL of 1.0 M hydrochloric acid. What is the pH of the resulting solution?20. Properties of a Buffer The amino acid glycine is often used as the main ingredient ofbuffer in biochemical experiments. The amino group of glycine, which has a pKa of 9.6, can exist either in the protonated form or as the free base (—NH2), because of the reversible equilibrium R-NH3+  R- NH2 + H+(a) In what pH range can glycine be used as an effective buffer due to its amino group? (b) In a 0.1 M solution of glycine at pH 9.0, what