Question

# For each of the following solutions, calculate the initial pH and the final pH after adding...

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.

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

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

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