S11Q3W4 - CHEM 0120 SPRING 2011 Monday 1:00 PM Golde QUIZ 3 WEEK 4 Name For ax 2 bx c = 0 x = ± b b ac a 2 4 2 K w = 1.00 x 10-14 1 A weak acid

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Unformatted text preview: CHEM 0120 SPRING 2011 Monday 1:00 PM Golde QUIZ 3 WEEK 4 January 24, 2011 Name_______________________________ For ax 2 + bx + c = 0, x = - ±- b b ac a 2 4 2 ; K w = 1.00 x 10-14 1. A weak acid, HCN, reacts with a weak base, NH 3 , as follows: HCN(aq) + NH 3 (aq) NH 4 + (aq) + CN- (aq), K c = 0.755 By mixing these reagents, a solution was prepared which was initially 0.250M in HCN and 0.250M in NH 3 . Reaction then occurred until equilibrium was attained. Calculate the equilibrium concentrations of HCN and CN- in this solution. 2. A solution has a hydronium ion concentration, [H 3 O + ] = 2.58 x 10-4 M. Determine the pH and the pOH of this solution. ANSWERS 1. HCN + NH 3 NH 4 + + CN- Initial 0.250 0.250 0 Change -x -x +x +x Equilib 0.250 - x 0.250 - x x x K c = +- 2 4 2 3 [NH ][CN ] (x) [HCN][NH ] (0.250 - x) = = 0.755 Take square roots of both sides: 1/2 x = (0.755) 0.250 - x = 0.869 0.2172 – 0.869x = x; x = 0.116 {Or use SOLVER} [HCN] = 0.250 – 0.116M = 0.134M; [CN- ] = 0.116M 2. pH = -log 10 (2.58 x 10-4 ) = 3.588 pOH = 14.00 – pH = 10.41 CHEM 0120 SPRING 2011 Monday 6:00 PM Golde QUIZ 3 WEEK 4 January 24, 2011 For Make-up Only Regular recitation Day: Time: instructor: Quiz Score: Group score: Scores recorded by: Name_______________________________ For ax 2 + bx + c = 0, x = - ±- b b ac a 2 4 2 ; K w = 1.00 x 10-14 1. Consider the reaction C(s) + CO 2 (g) 2 CO(g), which has an equilibrium constant, K p = 7.23 at a certain temperature. a) A graphite container was filled with CO 2 gas. When equilibrium was attained, the pressure of CO 2 was 2.65atm. Determine the pressure of CO at equilibrium. b) The temperature was then changed by a small amount, and K p at the new temperature was 11.7. As a result of the temperature change, in which direction will the reaction occur (left- to-right or right-to-left) in order to restore equilibrium? A student determined that the new equilibrium pressure of CO 2 is 2.97atm. Do you agree with her result? Give your reasoning. 2. The following solutions, 0.10M HBr, 0.10M NH 3 and 0.10M HNO 2 , have the following pH values (but not necessarily in the order given): 11.1, 2.95, 1.00. Match each pH value with the correct solution. Give your reasoning. ANSWERS 1. a)At equilibrium, ) p(CO p(CO) 2 2 = K p . So p(CO) 2 = 7.23 x 2.65 = 19.16; p(CO) = 4.38atm b) As K p is increased by the change in temperature, the reaction must occur left-to-right , i.e. CO 2 is consumed and CO is formed. It is therefore impossible for the new CO 2 pressure to be 2.97atm. 2. HBr is a strong acid and has the lowest pH, 1.00 HNO 2 is a weak acid and has a somewhat higher pH, but it must be below 7: 2.95 NH 3 is a weak base, and so will have a pH greater than 7: 11.1 CHEM 0120 SPRING 2011 Tuesday 8:00 AM Golde QUIZ 3 WEEK 4 January 25, 2011 Name_______________________________ K w = 1.00 x 10-14 For Make-up Only Regular recitation Day: Time: instructor: Quiz Score: Group score: Scores recorded by: 1. Given the following equilibria and K1....
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This note was uploaded on 02/20/2011 for the course CHEM 110 taught by Professor Dr.huston during the Spring '06 term at Pittsburgh.

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S11Q3W4 - CHEM 0120 SPRING 2011 Monday 1:00 PM Golde QUIZ 3 WEEK 4 Name For ax 2 bx c = 0 x = ± b b ac a 2 4 2 K w = 1.00 x 10-14 1 A weak acid

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