5. Ch 8: Use a systematic treatment of equilibrium to calculate the pH of a solu

Question

5. Ch 8: Use a systematic treatment of equilibrium to calculate the pH of a solution prepared by diluting 0.467 g of aniline (aminobenzene, C6H5NH2) to 100.0 mL. Pertinent equilibria include the dissociation of the weak base and the autoprotolysis of water. a. Write out the mass balance and charge balance equations. b. Write the associated equations for the equilibrium constants for the pertinent equilibria. c. Make the assumption that [BH+] ~[OH-] and solve for [OH-]. Calculate the pH of the solution. Verify that the assumption was justified. Calculate the fraction of dissociation a.

Explanation / Answer

a) C6H5NH2(aq) + H2O -----------> C6H5NH3+(aq) + OH-(aq) , aniline moles = 0.467/93.13 = 0.005mols = 0.0005 M, at equilibrium aniline moles = 0.005-x , anilinium ion moles = x = OH- moles, their respective conc wil be obtaoined by dividing vol , b) equi const K=[Anilinium ion][OH-]/[Aniline] = 10^ -9.3 = 5.012 x10^ -10 , c) 5.012 x10^ -10 = x^2/(0.005-x) , x= 5 x10^ -7 =[OH-] , pOH = -log(5 x10^ -7) = 6.3 , OH_ moles = 5 x10^ -7 x 1000/100 = 5 x10^ -6 , fraction of diisoiation =( 5 x10^ -6/0.005) x100 = 0.1 %

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