probset3withanswers

probset3withanswers - Chem33-1, C-hour Problem Set 3...

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Chem33-1, C-hour Problem Set 3 Due Date: Oct. 19, 2007 Student name: ___________________ Student SISD #: __________________ 1. Classify the following as strong or weak acids or bases in aqueous solution a. HNO 2 b. HNO 3 c. HCl d. HF ANSWER: Strong acids have a K a > > 1 and weak acids have K a < 1. Table 7.2 in the text lists some K a values for weak acids. K a values for strong acids are hard to determine so they are not listed in the text. However, there are only a few common strong acids so if you memorize the common strong acids, then all other acids will be weak acids. The strong acids to memorize are HCl, HBr, HI, HNO 3 , HClO 4 and H 2 SO 4 . a. HNO 2 is a weak acid (K a = 4.0 x 10 -4 ). b. HNO 3 is a strong acid. c. HCl is a strong acid. d. HF is a weak acid (K a = 7.2 x 10 -4 ). 2. The hydride ion (H - ) and methoxide ion (CH 3 O - ) have much greater affinity for H + the OH - ion. Write equations for the reactions that occur when NaH and NaOCH 3 are dissolved in water. ANSWER: H - (aq) + H 2 O(l) H 2 (g) + OH - (aq); OCH 3 - (aq) + H 2 O(l) CH 3 OH(aq) + OH - (aq)
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3. Consider the acid-base reaction HF (aq) + H 2 O (l) F - (aq) + H 3 O + (aq) at a temperature of 298K. From tables one knows the following Gibbs free energies of formation: HF (aq): Δ G f = -296.82 kJ/mol H 2 O (l): G f = -237.18 kJ/mol F - (aq): G f = -278.79 kJ/mol H 3 O + (aq): G f = -237.18 kJ/mol a. Calculate the Gibbs free energy for the reaction HF (aq) + H2O (l) F- (aq) + H3O+ (aq) b. Calculate the equilibrium constant, K. c. Calculate the pKa of hydrofluoric acid. ANSWER: a. Δ Δ = Δ ts reac f products f r G G G tan ν ( ) ( ) ( ) ( ) HF G O H G O H G F G f f f f Δ Δ Δ + Δ = + 2 3 ) 82 . 296 ( ) 18 . 237 ( ) 18 . 237 ( 79 . 278 + = mol
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probset3withanswers - Chem33-1, C-hour Problem Set 3...

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