University of Texas CH 302 McCord H07 - Acid-Base 2

University of Texas CH 302 McCord H07 - Acid-Base 2 -...

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Unformatted text preview: byrne (cmb3744) – H07: Acid-Base 2 – mccord – (51620) 1 This print-out should have 21 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Assume that five weak acids, identified only by numbers (1, 2, 3, 4, and 5), have the following ionization constants. Ionization Acid Constant K a value 1 1 . × 10 − 3 2 3 . × 10 − 5 3 2 . 6 × 10 − 7 4 4 . × 10 − 9 5 7 . 3 × 10 − 11 The anion of which acid is the strongest base? 1. 1 2. 4 3. 2 4. 3 5. 5 correct Explanation: 002 10.0 points What is the percent ionization for a weak acid HX that is 0.40 M? K a = 4 . × 10 − 7 . 1. 0.050% 2. 0.020% 3. 2.0% 4. 0.00020% 5. 0.10% correct Explanation: 003 10.0 points Like all equilibrium constants, K w varies somewhat with temperature. Given that K w is 1 . 8 × 10 − 13 at some temperature, compute the pH of a neutral aqueous solution at that temperature. Correct answer: 6 . 37236. Explanation: K w = 1 . 8 × 10 − 13 K w = [H 3 O + ][OH − ] = 1 . 8 × 10 − 13 In a neutral solution [H 3 O + ] = [OH − ] [H 3 O + ] = radicalbig 1 . 8 × 10 − 13 = 7 . × 10 − 7 pH = − log [H 3 O + ] = 6 . 37236 004 10.0 points A 0.00100 M solution of a weak acid HX is 10% ionized. Calculate K a for the acid. Correct answer: 1 . 11111 × 10 − 5 . Explanation: 005 10.0 points The term “ K a for the ammonium ion” de- scribes the equilibrium constant for which of the following reactions? 1. NH 3 + H 3 O + ⇀ ↽ NH + 4 + H 2 O 2. NH + 4 + H 2 O ⇀ ↽ NH 3 + H 3 O + correct 3. NH + 4 + OH − ⇀ ↽ NH 3 + H 2 O 4. The term is misleading, because the am- monium ion is not an acid. 5. NH 3 + H 2 O ⇀ ↽ NH 4 + + OH − 6. NH 4 Cl(solid) + H 2 O ⇀ ↽ NH + 4 + Cl − Explanation: 006 10.0 points Hydroxylamine is a weak molecular base with byrne (cmb3744) – H07: Acid-Base 2 – mccord – (51620) 2 K b = 6 . 6 × 10 − 9 . What is the pH of a 0.0500 M solution of hydroxylamine? 1. pH = 4.74 2. pH = 8.93 3. pH = 7.12 4. pH = 10.37 5. pH = 3.63 6. pH = 9.26 correct 7. pH = 9.48 Explanation: Hydroxylamine is a weak base, so use the equation to calculate weak base [OH − ] con- centration (note that this is the approximate equation. Why? Because K b is very small and the concentration is reasonable) : [OH − ] = radicalbig K b C b = radicalBig (6 . 6 × 10 − 9 ) (0 . 0500)...
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University of Texas CH 302 McCord H07 - Acid-Base 2 -...

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