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solutionEXAM2_pdf

# solutionEXAM2_pdf - Version 041 Exam 2 Fakhreddine(52395...

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Version 041 – Exam 2 – Fakhreddine – (52395) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Suppose the reaction H 2 (g) + I 2 (g) 2 HI(g) has an equilibrium constant K c = 49 and the initial concentrations of H 2 , I 2 and HI are 0.50 M, 0.50 M and 0.00 M, respectively. What is the correct value for the final concentration of HI(g)? 1. 0.599 M 2. 0.778 M correct 3. 0.389 M 4. 0.250 M 5. 0.438 M 6. 0.219 M Explanation: K c = 49 [H 2 ] ini = 0 . 50 M [I 2 ] ini = 0 . 50 M [HI] ini = 0 . 0 M H 2 (g) + I 2 (g) 2 HI (g) ini, M 0.5 0.5 0 Δ, M x x 2 x eq, M 0 . 5 x 0 . 5 x 2 x K c = [HI] 2 [H 2 ] [I 2 ] = 49 (2 x ) 2 (0 . 50 x ) 2 = 49 2 x 0 . 50 x = 7 2 x = 3 . 5 7 x 9 x = 3 . 5 x = 0 . 389 [HI] = 2 x M = 0 . 778 M 002 10.0 points K = 8, for the gas phase reaction N 2 O 4 2 NO 2 . If 2 moles of N 2 O 4 are put into a 1 liter vessel, the number of moles of NO 2 at equilib- rium ( n x ) can be calculated from the equation 1. 8 = n 2 x 2 n x 2 correct 2. 8 = n x 2 n x 2 3. 8 = n 2 x 2 n x 4. 8 = n 2 x 2 5. 8 = 2 n 2 x 2 n x Explanation: Since the problem asks you to put the equa- tion in terms of n x , which we are told is equal to [NO 2 ], we need a slight modification to our normal way of working this problem. Working this problem the normal way tells us that final [NO 2 ] is equal to 2 n x . Continuing with this method will not give us the correct answer. Normal way: N 2 O 4 2 NO 2 (g) Initial 2 0 Change n x +2 n x Equilibrium 2 n x 2 n x In the modified way, all n x coefficients are divided in half to give the appropriate [NO 2 ] = n x equality. This is ok to do because we’ve kept the correct ratio in our change step; i.e. , [NO 2 ] must be twice as big as [N 2 O 4 ]. Modified way: N 2 O 4 2 NO 2 (g) Initial 2 0 Change 0 . 5 n x + n x Equilibrium 2 0 . 5 n x n x Continuing with the modified way . . .

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Version 041 – Exam 2 – Fakhreddine – (52395) 2 K = 8 = [NO 2 ] 2 [N 2 O 2 ] = n 2 x 2 0 . 5 n x = n 2 x 2 n x 2 003 5.0 points In a solution labeled “0.10 M HNO 3 ,” which of the following is CORRECT? 1. [HNO 3 ] = 0.050 M, [H 3 O + ] = 0.050 M, [NO 3 ] = 0.050 M 2. [H 3 O + ] = 0.10 M, [OH ] = 1 . 0 × 10 7 M 3. [HNO 3 ] = 0.10 M 4. [H 3 O + ] = 0.09 M, [NO 3 ] = 0.010 M 5. [H 3 O + ] = 0.10 M, [NO 3 ] = 0.10 M cor- rect Explanation: 004 5.0 points Which of the following is the best chemical definition of pure (or neutral) water at any temperature? 1. water that has a pH equal to ZERO 2. water where the concentration of H + is equal to the concentration of OH correct 3. water that has a pH exactly equal to 7.00 4. water that has been freshly pumped from a well 5. water that has a pH equal to K w 6. water that has a pOH slightly above 7.00 Explanation: You were probably able to narrow this one down to either “water where the concentra- tion ...” or “water that has a pH exactly equal to 7.00.” The second of these is incorrect because pH varies with temperature since K w varies, but the concentrations of H and OH will still be equal to each other.
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solutionEXAM2_pdf - Version 041 Exam 2 Fakhreddine(52395...

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