a4s-1 - Chemistry 2000 Spring 2001 Section B Assignment 4...

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Unformatted text preview: Chemistry 2000 Spring 2001 Section B Assignment 4 Solutions (a) The solubility equilibrium is Pb2aq ¦ § ¨£ ¢   L.   © 4 mol ©  ©     (b) In a pH 4 buffer, aH 10 10 10 . Since aMg2 Using Kw , we find that aOH Ksp a2 , we find a solubility of 1 5 109 mol L. This absurdly high number indicates OH that the solubility will be extremely high, and mostly limited by the buffering capacity of the solution.  ©   4. 10  ¡ The solubility is therefore 1 55 2s. s 2s 2 4s3 1 55 10 4 11 s 2 s and aOH   10 aOH Thus  15 ¡ Suppose that the solubility is s. Then aMg2  aMg2 11 . 10 © The solubility product (Ksp ) of this salt is 1 5 Ksp 2OH aq  2s  ¡ ¤ ¥£ ¢ ¡ Mg OH £ ¢© 1.   ©   2. We can use the molar mass to convert the solubility to mol/L: 4 mol L 3F aq  £ ¢©    Ga3aq §£ ¢ ¦     GaF3 s 10 © The solubility process is 16  0 02 g L 126 71 g mol  s ¤ £ ¢ The concentration of gallium in solution will be equal to s while the concentration of fluoride will be 3s. Accordingly, 10 14 ¡  (a) 1 kg of maple sugar is 2% of the initial weight of sap, i.e. 0 02m that m 50 kg.   1  17  27s4  3  3. s 3s © Ksp 1 kg, which implies (b) We need to take the sap from 5 C to the boiling point, assumed to be about 100 C.1 The amount of heat required to do this is  1 ¡© 103 g 4 18 J K 1g 95 K 20 MJ ¡ ©  ¡ 50   ˜ mCP ∆T    q1 We must also boil away 49 kg of water. The heat of vaporization of water is 2260 J/g, so that takes q2 49 103 g 2260 J g 111 MJ ¡    4 MJ kg  ¡  33 kg. (a) We need a balanced reaction: H2 O l 1 H 2 2g NaOH aq §£ ¢ Na s £ ¢ 4. 131 MJ ¡ (c) mwood  ¡ The total heat required is therefore 131 MJ. ! "£ ¢ §£¢ For this reaction, ¯ ∆H f   § % § 1% ¡ 0 § £$ # % &£ $ # ¢  g s ¯ ∆H f H2 O l 285 830 kJ mol  2 3¡  0 Na § £$ # ¢ 1 470 114 0 2 184 284 kJ mol H2 ( )£ $ # ' 1¯ ∆H f 2 NaOH aq ¢ ¯ ∆H f ¢  %   %    ¯ ∆H The number of moles of sodium is % ¡   184 284 kJ mol 0 43 mol 80 kJ    ¡   %  q  The total heat released is therefore 0 43 mol  10 g 22 9898 g mol  mNa (b) The reaction is exothermic so the water gains 80 kJ of heat.  ©  ¡ 48 C 48 K 63 C.    §  The final temperature would be 15 1  80 103 J 400 g 4 18 J K 1g © ¡ ∆T (c) The reaction produces 0.43 mol of hydroxide ions. The volume of the solution is 400 g 0 9971 g mL 401 mL. The concentration of hydroxide is therefore 0 43 mol 0 401 L 1 1 mol L. The activity of hydrogen ions would be approxi14 1 1 mately 10 9 2 10 15 , which gives a pH of 14.0.  © ¡   ¡     ©  4¡ ¡  1 This is probably the shakiest assumption in the whole calculation since the boiling point will increase as the sap becomes more concentrated. 2 ...
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This note was uploaded on 03/03/2012 for the course CHEM 2000 taught by Professor Roussel during the Fall '06 term at Lethbridge College.

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