HW1 Ans - Water l Solubility of Ethanol in Water Explain...

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Water l. Solubility of Ethanol in Water Explain why ethanol (CHBCH2OFI) is more soluble in water than is ethane (CH3CH3). Answer Ethanol is polar; ethane is not. The ethanol -OH group can hydrogen-bond with water. 2. Calculation of pH from Hydrogen Ion Coneentration What is the pH of a solution that has an H+ concentration of (a) I.75 x 10-5 mol/L; (b) 6.50 x 10-10 moVL; (c) 1.0 x l0-4 mol/L; (d) 1.50 x 10-5 mol/L? Answer Using pH = -log [H*]: (a) -log (1.?5 x 10-5) : 4.76; (b) -log (6.50 x 10-10) 9.19;(c) -log (1.0 x 10-a; 4.6' (d) -log (1.50 x 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+] - IO-PH: [H*] :10-3'82:1.51 x 10-4M;(b) 1g-6'sz =3.02 x l0-7u; (c) 10-rr'rr :7.76 x 1o-12 rrl. H 4. Acidity of Gastric HCI In a hospital laboratory a 10.0 mL sample of gastric juice, obtained several rr hours after a meal, was titrated with 0.1 rrr 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 r is the concentration of gastric HCI (mol,/L), (0.010 L)x: (0.0072 L)(0.1 moVL) n 0.072 tr.l gastric HCI Given that pH and that HCI is a strong acid, pH: -log (7.2x 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 x 10-a lt HCl. (c) Write out the acid dissociation for sodium hydroxide. (d) Calculate the pH of a solution of 7.0 x 10-5 lr NaOH. s-13
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S-14 Chapter 2 ttlater Answer (a) HCI -.- H+ + Cl- (b) HCI is a strong acid and firlly dissociates into H+ and Cl-. Thus, [H*] : [Cl-] = tHCll. pH : -log -log (5.0 x 10-4 u) 3.3 (two significant figures) (c) NaOH -.- Na+ + OH- (d) is a strong base; dissociation in aqueous solution is essentially complete, so lNa+1 :[OH-1 :[NaOH]. PH+POH:14 pOH: -log [OH-] pH t4 + log 14 + log (7.0 x 10-5) 9.8 (two signilicant Calculation of pH from Concentration of Strong Acid Calculate the pH of a solution prepared by diluting 3.0 mL of 2.5 w HCI to a final volume of 100 mL with HzO. Answer Because HCI is a strong acid, it dissociates completely to H* + Cl-. Therefore, 3.0 mL x 2.5 M HCI 7.5 meq of H*. In 100 mL of solution, this is 0.075 ru H+. pH: -log : -log (0.075) : -(-1.1) : 1.1 (twosignilicantfigures) Measurement of Acetyleholine 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 enz],rne acetylcholinesterase, acetylcholine is quantitatively converted into choline and acetic acid, which dissociates to yield acetate and a hydrogen ion: 6. t. o 9H' cH.-C-o-cH2-cH2-*rf-cH. I'o- -l Acetylcholine CH' ?"' HO-CH2-CH2-*T-CH3 + CH, Choline : CH3-C-O- + 11* tl o Acetate 'J In a typical analysis, 15 mL of an aqueous solution containing an unknown amount Qf bcetylcholine had a pH of 7.65. When incubated with acetylcholinesterase, the pH of the solution decr64sed to 6.87. As- suming that there was-no buffer in the assay mixture, determine the number of moles of acetylcholihe in the 15 mL sample.
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This note was uploaded on 10/22/2009 for the course CHEM 140A taught by Professor Whiteshell during the Spring '04 term at UCSD.

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HW1 Ans - Water l Solubility of Ethanol in Water Explain...

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