Wright CHM 1220 - CHM 1220_Exam3Review_InClass_Fall2017_answers (2 pages)

Unformatted text preview:

CHM 1220 In Class Activity Aqueous Equilibria Complex Ions and Electrochemistry Fall 2017 ANSWERS 1 A 25 0 mL solution of quinine was titrated with 1 00 M hydrochloric acid HCl It was found that the solution initially contained 0 125 moles of quinine What was the pH of the solution after 75 00 mL of the HCl were added Quinine is monobasic with pKb 5 10 quinine H aq OH aq QH 1 00 mol HCl Need to determine where in the titration we are 75 00 mL HCl soln 0 075 mol HCl added by comparing moles of analyte and titrant 1000 mL soln HCl Initially quinine aq H2O l Q mol analyte does not equal mol titrant 0 125 mol quinine 0 05 mol HCl added NOT at equivalence point or start can use Henderson Hasselbalch equation pH pK a log Q QH Need concentrations or mol since volume identical for Q and QH mol HCl added mol QH formed 0 075 mol QH mol QH formed mol Q consumed 0 075 mol mol Q remaining 0 125 mol Q initially 0 075 mol Q consumed 0 05 mol Q Plug in values solve for pH pH pK a log 2 Q QH 0 05 mol 14 5 10 log 8 90 0 176 8 72 0 075 mol Calculate Ecell for an electrochemical cell based on the two half reactions given below when the reaction occurs in an acidic aqueous solution in which the pH is buffered at 7 000 and BrO 3 0 880 M Cl 0 144 M and 0 260 atm Use the smallest wholenumber coefficients possible when balancing the overall reaction BrO3 aq 6 H aq 5 e Br2 3 H2O E 1 52 V reduction at cathode Cl2 g 2 e 2 Cl aq E 1 36 V oxidation at anode Net redox reaction 2 BrO3 aq 12 H aq 10 Cl aq Br2 6 H2O 5 Cl2 g Number of electrons transferred n 10 mol e Non standard conditions need Nernst equation to calculate cell potential Standard cell potential o o o Ecell Ecathode Eanode E o cell 1 52 V 1 36 V o Ecell 0 16 V Ecell E o cell Plug in values 0 260 0 0592 0 16 V log 10 0 880 2 1 10 7 12 0 144 10 5 Ecell Cl PCl2 0 0592 0 0592 o log Q Ecell log 2 n n BrO3 H Ecell 0 16 0 530 0 37 V 5 12 10 CHM 1220 3 In Class Activity Aqueous Equilibria Complex Ions and Electrochemistry What mass of Ca NO3 2 must be added to 1 0 L of a 1 0 M HF solution to begin precipitation of CaF2 s For CaF2 Ksp 4 0 10 11 and Ka for 7 2 10 4 Assume no volume change on addition of Ca NO3 2 s Dissolution of Ca NO3 2 Ca NO3 2 s Ca2 aq 2 NO3