Question

Use the Henderson-Hasselbach equation to calculate the mass of solid sodium acetate required to mix with 50.0 mL of 0.10 M acetic acid to prepare a pH 4 buffer. Ka for acetic acid is 1.8 x 10^-5.

Answer 1

Given that the volume of the solution = 50 mL = 0.0500 L Conc. of HCH3CO2 = 0.10 M K, of HCH2CO2 = 1.8 x 10-5 pk = -log Ka = -log (1.8 x 10-5) = 4.745 pH of the solution = 4.00 Molecular weight of NaCH3CO, = 82.03 g/mol We know that pH of the buffer is calculated by using Henderson's equation, [base] pH = pka+log [acid] [CH,CO,-) 34.00 = 4.745 +log: ® [HCH,CO,] [CH,CO, 1--0.745 log inCH.CO2] [CH,CO, ] -10-0.745 = 0.18 [HCH,CO2] [CH,CO,-]=0.18x[HCH,CO,]=0.180.10M =0.018M Assumed that there is no change in volume by the addition of NaCH.CO, Volume of the solution = 0.0500 L Number of moles of NaCH,CO, = Molarity x volume = 0.018Mx0.0500L = 0.0009 mol mass of NaCH,CO2 = moles x molecualr weight = 0.0009 mol x82.03 g.mol-1 = 0.073827 g Mass of NaCH,CO, = 0.0738 g