Question

. What mass of A chemistry graduate student is given 450 mL of a 0.90 M pyridine (CHN) solution. Pyridine is a weak base with K, -1.7*10 CH NHCI should the student dissolve in the CHN solution to turn it into a buffer with pH = 5.007 You may assume that the volume of the solution doesn't change when the C H NHCl is dissolved in it. Be sure your answer has a unit symbol, and round it to 2 significant digits

Answer 1

Kb = 1.7x100 pk = -log[ Kb ] pkb = - log ( 1.7x109 ] pKb = 8.77 pOH = 14 - ph pOH = 14 - 5.00 pOH = 9.00 Calculating number of moles: molariy of C5H5N = 0.90 M. volume of C5H5N = 450 mL = 0.45 L number of moles of C5H5N = ? number of moles of C5H5N = Molarity * volume number of moles of C5H5N = 0.90 X 0.45 number of moles of C5H5N = 0.405 mol

weak base = C5H5N salt = C5H5NHCI C5H5N p Kb = 8.77 [C5H5NHCI ] = ? [ C5H5N ] = 0.405 mol pOH = 9.00 According to Henderson-Hasselbalch equation [ salt pOH = pkb + log – [ base [ C5H5NHCI 9.00 = 8.77 + log [ 0.405 [C5H5NHCI ] = 0.689 mol 1 1 ] Calculating mass: mole of C5H5NHCl = 0.689 mol molar mass of C5H5NHC = 115.56 g/mol mass of C5H5NHCl = mole x molar mass mass of C5H5NHCl = 0.689 115.56 mass of C5H5NHCl = 79.56 g