Question

Find the pH of 500.0 mL of the buffer from excerise 6 (Find the pH of a buffer composed of 0.80 M ammonia and 0.50 ammonium chloride... PH 9.4) after the addition of 100.0 mL of 0.3 M barium hydroxide.

Answer 1

initially

pH = pKb + log [NH4Cl] / [NH3]

pKb of NH3 = 4.75

pOH = 4.75 + log [0.5] / [0.8]

pOH = 4.54

pH = 14 - 4.54

pH = 9.46

after Ba(OH)2 added

initial millimoles of NH3 = 500 x 0.8 = 400

millimoles of NH4Cl = 500 x 0.5 = 250

millimoles of OH^-  added = 2 (100 x 0.3) = 60

now

millimoles of NH3 = 400 + 60 = 460

millimoles of NH4Cl = 250 - 60 = 190

total volume = 500 + 100 = 600

[NH3] = 460 / 600 = 0.77 M

[NH4Cl] = 190 / 600 = 0.32 M

pOH = 4.75 + log [0.32] / [0.77]

pOH = 4.37

pH = 14 - 4.37

pH = 9.63