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pob5e_solutions_ch02 - 2608T_ch02sm_S13-S25 5:57PM Page...

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S-13 Water 1. Solubility of Ethanol in Water Explain why ethanol (CH 3 CH 2 OH) is more soluble in water than is ethane (CH 3 CH 3 ). Answer Ethanol is polar; ethane is not. The ethanol —OH group can hydrogen-bond with water. 2. 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? Answer Using pH log [H ]: (a) log (1.75 10 5 ) 4.76; (b) log (6.50 10 10 ) 9.19; (c) log (1.0 10 4 ) 4.0; (d) log (1.50 10 5 ) 4.82. 3. 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? Answer Using [H ] 10 pH : (a) [H ] 10 3.82 1.51 10 4 M ; (b) [H ] 10 6.52 3.02 10 7 M ; (c) [H ] 10 11.11 7.76 10 12 M . 4. 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 pa- tient’s stomach contained no ingested food or drink, thus assume that no buffers were present. What was the pH of the gastric juice? Answer Multiplying volume (L) by molar concentration (mol/L) gives the number of moles in that volume of solution. If x is the concentration of gastric HCl (mol/L), (0.010 L) x (0.0072 L)(0.1 mol/L) x 0.072 M gastric HCl Given that pH log [H ] and that HCl is a strong acid, pH log (7.2 10 2 ) 1.1 5. 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. chapter 2 2608T_ch02sm_S13-S25 2/1/08 5:57PM Page S-13 ntt 102:WHQY028:Solutions Manual:Ch-02:
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S-14 Chapter 2 Water Answer (a) HCl H Cl (b) HCl is a strong acid and fully dissociates into H and Cl . Thus, [H ] [Cl ] [HCl]. pH log [H ] log (5.0 10 4 M ) 3.3 (two significant figures) (c) NaOH Na OH (d) NaOH is a strong base; dissociation in aqueous solution is essentially complete, so [Na ] [OH ] [NaOH]. pH pOH 14 pOH log [OH ] pH 14 log [OH ] 14 log (7.0 10 5 ) 9.8 (two significant figures) 6. 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 H 2 O. Answer Because HCl is a strong acid, it dissociates completely to H Cl . Therefore, 3.0 mL 2.5 M HCl 7.5 meq of H . In 100 mL of solution, this is 0.075 M H . pH log [H ] log (0.075) ( 1.1) 1.1 (two significant figures) 7. 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 quantitatively converted into choline and acetic acid, which dissociates to 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. As- suming that there was no buffer in the assay mixture, determine the number of moles of acetylcholine in the 15 mL sample.
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