Question

For each of the following solutions, calculate the initial pH and the final pH after adding 0.010 mol of NaOH.

Part A: For 270.0 mL of a buffer solution that is 0.205 M in HCHO2 and 0.305 M in KCHO2, calculate the initial pH and the final pH after adding 0.010 mol of NaOH. Express your answers using two decimal places separated by a comma.

Part B: For 270.0 mL of a buffer solution that is 0.285 M in CH3CH2NH2 and 0.260 M in CH3CH2NH3Cl, calculate the initial pH and the final pH after adding 0.010 mol of NaOH. Express your answers using two decimal places separated by a comma.

Answer 1

part A

pH of formicacid buffer = pka + log( KCHO2/HCHO2)

pka of formicacid = 3.75

pH = 3.75+log(0.305/0.205)

initial pH = 3.92

after addition of NaOH

concentration of NaOH added = 0.01/0.27 = 0.037 M

pH of formicacid buffer = pka + log( KCHO2+NaOH/HCHO2 - NaOH)

final pH = 3.75+log((0.305+0.037)/(0.205-0.037))

          = 4.06
Part B

pH of CH3CH2NH2 buffer = 14 - (pkb + log(CH3CH2NH3Cl/CH3CH2NH2))

pkb of CH3CH2NH2 = 3.367

pH = 14 - (3.367+log(0.26/0.285))

initial pH = 10.67

after addition of NaOH

concentration of NaOH added = 0.01/0.27 = 0.037 M

pH of CH3CH2NH2 buffer = 14 - (pkb + log(CH3CH2NH3Cl - NaOH/CH3CH2NH2+NaOH))

      = 14 - (3.367+log((0.26-0.037)/(0.285+0.037))

     final pH = 10.79