Question

A chemistry graduate student is given 500. mL of a 0.30 M acetic acid (HCH,CO2) solution. Acetic acid is a weak acid with Ka=1.8 10 . What mass of KCH, CO, should the student dissolve in the HCH2CO2 solution to turn it into a buffer with pH =5.03? You may assume that the volume of the solution doesn't change when the KCH CO, is dissolved in it. Be sure your answer has a unit symbol, and round it to 2 significant digits.

Answer 1

Given that the volume of the solution = 500 mL = 0.500 L Conc. of HCH3CO2 = 0.30 M K, of HCH2CO2 = 1.8 x 10-5 pk = -log Ka = -log (1.8 x 10-5) = 4.745 pH of the solution= 5.03 Molecular weight of KCH,CO, = 98.14 g/mol We know that pH of the buffer is calculated by using Henderson's equation, **Pka+log [base] 10% [acid] 35.03 = 4.745 +logo [CH,CO, ] [HCH,CO,] [CH,CO, ] -0.285 [HCH,CO,] [CH,CO, 1 -100.285 = 1.9287 [HCH,CO,] [CH,CO, ]=1.9287x [HCH,CO,]=1.9287x0.30M =0.57862M Assumed that there is no change in volume by the addition of KCH,CO, Volume of the solution = 0.500 L Number of moles of KCH,CO, =Molarity x volume =0.57862M *0.500L =0.28931 mol mass of KCH,CO, = moles x molecualr weight =0.28931 mol x 98.14 g.mol-? = 28.3929 g Mass of KCH,CO, = 28 g