What are the concentrations of OH-(aq), (CH3)2NH2+(aq), and (CH3)2NH in a 0.450M solution of dimethylamine? Kb = 7.40 x 10-4                                                            (CH3)2NH + H2O  ↔ (CH3)2NH2+ + OH I 0.450 0 0 C -x +x +x E 0.450 - x

Define or illustrate the meaning of the following terms: a. Ka reaction b. Ka equilibrium constant c. Kb reaction d. Kb equilibrium constant e. conjugate acidbase pair

16-27 Guanidine, p/Ca 13.6, is a very strong base, almost as basic as hydroxide ion. NH NH2+ II II H2N—C—NH2 + H2O H2N—C—NH2 + OH" Guanidine Guanidinium ion pKb = 13.6 Complete the Lewis structure for guanidine, showing all valence electrons. The remarkable basicity of guanidine is attributed to the fact that the positive charge on the guanidinium ion is delocalized by resonance over the three nitrogen atoms. This delocalization increases the stability of the guanidinium ion relative to the ammonium ion or substituted ammonium ions. Draw three equivalent contributing structures for the guanidinium ion and show by the use of curved arrows how these three contributors are related. Propose an explanation for the fact that protonation occurs on the C=NH nitrogen rather than on one of the —NH., nitrogens. (Hint: Consider the resonance stabilization of the structure formed by protonation on a —NH., nitrogen compared with the resonance stabilization of the structure formed by protonation on the =NH nitrogen.) Predict the N—C—N bond angles in the hybrid. Which is the stronger acid, the ammonium ion or the guanidinium ion?

Calculate [OH], [H+], and the pH of 0.40 M solutions of each of the following amines (the Kb values are found in Table 13-3). a. aniline b. methylamine