Exam 2-solutions

# Exam 2-solutions - Version 034 Exam 2 fakhreddine(51650...

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Version 034 – Exam 2 – fakhreddine – (51650) 1 This print-out should have 26 questions. Multiple-choice questions may continue on the next column or page – Fnd all choices before answering. 001 10.0 points What is the concentration of OH ions in a 0.40 M solution of KCN? The ionization constant of HCN is 4 . 0 × 10 10 . 1. 6 . 25 × 10 5 2. 1 . 60 × 10 10 3. 3 . 16 × 10 3 correct 4. 1 . 26 × 10 5 5. 4 . 08 × 10 12 Explanation: 002 10.0 points Solution A contains 0.5 grams of solute A and solution B contains 0.5 grams of solute B (both A and B are nonelectrolytes). Other than the solutes, the solutions are identical (volume, temperature, etc .). Now you mea- sure the osmotic pressure of each solution and Fnd that the osmotic pressure of solution B is twice that of solution A. What is the relation- ship between the molecular weights of solutes A and B? 1. The molecular weight of solute B is four times that of solute A. 2. The molecular weight of solute A is twice that of solute B. correct 3. The molecular weight ratio cannot be determined from this experiment. 4. The molecular weight of solute B is twice that of solute A. 5. The molecular weight of solute A is four times that of solute B. Explanation: Because the solution with B has twice the osmotic pressure, it must also have twice the number of dissolved particles (number of moles of B). The only way to have more moles for B (knowing the same mass of A and B was used) is that B has a smaller molecular weight than A. ±or the osmotic pressure to be ex- actly twice that of A, the molecular weight of B must be half that of A which is the same as A being twice that of B. 003 10.0 points Assume that Fve weak acids, identiFed only by numbers (1, 2, 3, 4 and 5), have the follow- ing ionization constants. Ionization Acid Constant K a value 1 1 . 0 × 10 3 2 3 . 0 × 10 5 3 2 . 6 × 10 7 4 4 . 0 × 10 9 5 7 . 3 × 10 11 A 0.10 M solution of which acid would have the highest pH? 1. 4 2. 3 3. 5 correct 4. 2 5. 1 Explanation: K a = [H + ] [A ] [HA] HA H + + A A high pH means that the [H + ] concentra- tion is low. (Remember that values greater than 7 are basic!) The larger values of K a means that there is more [H + ] so you would expect these solutions to be more acidic; i.e. ,

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Version 034 – Exam 2 – fakhreddine – (51650) 2 have smaller pH’s. The smaller K a values mean less [H + ] in the solution, so higher pH’s. The acid with the smallest K a (#5) will have the highest pH and lowest [H + ] concentration. 004 10.0 points What would be the pH of a 0.25 M solution of phenylamine (C 6 H 5 NH 2 ) at room temper- ature? The K b of phenylamine is 4 × 10 10 . 1.
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## This note was uploaded on 07/25/2011 for the course CH 302 taught by Professor Rowland during the Spring '10 term at UT Arlington.

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Exam 2-solutions - Version 034 Exam 2 fakhreddine(51650...

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