Chem33-1, C-hour Problem Set 3

1. Classify the following as strong or weak acids or bases in aqueous solution a. b. c. d. HNO2 HNO3 HCl HF ANSWER: Strong acids have a Ka > > 1 and weak acids have Ka < 1. Table 7.2 in the text lists some Ka values for weak acids. Ka 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, HNO3, HClO4 and H2SO4. a. HNO2 is a weak acid (Ka = 4.0 x 10-4). b. HNO3 is a strong acid. c. HCl is a strong acid. d. HF is a weak acid (Ka = 7.2 x 10-4). 2. The hydride ion (H-) and methoxide ion (CH3O-) have much greater affinity for H+ the OH- ion. Write equations for the reactions that occur when NaH and NaOCH3 are dissolved in water. ANSWER: H-(aq) + H2O(l) → H2(g) + OH-(aq); OCH3-(aq) + H2O(l) → CH3OH(aq) + OH- (aq)