Question

8) Find the pH for 500.00mL of the buffer from exercise 5 after the addition of 100.00mL of 0.3M barium hydroxide. (Ex.5, Find the pH of a buffer composed of 0.80M ammonia and 0.90M ammonium chloride. Kb=1.8x10*-5 Answer for Ex. 5: pH= 9.21)

9)Find the pH of 500.00mL of the buffer from exercise 6 after the addition of 100.00mL of 0.3 barium hydroxide. (Ex.6, Find the pH of a buffer composed of 0.80M ammonia and 0.50M ammonium chloride. Answer Ex.6: pH=9.5)

11) If the pH of a buffer is supposed to be 2.5, what is a reasonable Ka for the weak acid used in the buffer? Many different values may be appropriate.

Answer 1

8)

initially,

moles of NH3= 0.80 M * 0.5 L = 0.40 mol

moles of NH4Cl= 0.90 M * 0.5 L = 0.45 mol

added mol of OH - =2*0.3 M * 0.1 L = 0.06 mol

so,

0.06 mol of NH4Cl will react to form additional 0.06 mol of NH3

after reaction,

mol of NH3 = 0.40 + 0.06 = 0.46 mol

mol of NH4Cl= 0.45 – 0.06 = 0.39 mol

pkb = -log Kb

    = -log (1.8*10^-5)

    = 4.745

use:

pOH = pKb + log {NH4CL/NH3}

pOH = 4.745 + log(0.39/0.46)

    = 4.67

PH = 14 – pOH

   = 14 – 4.67

   =9.33

Answer: 9.33

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