Design a buffer that has a pH of 6.95 using one of the weak base/conjugate acid

Question

Design a buffer that has a pH of 6.95 using one of the weak base/conjugate acid systems shown below.

Weak Base Kb Conjugate Acid Ka pKa

CH3NH2 4.2×10-4 CH3NH3+ 2.4×10-11 10.62

C6H15O3N 5.9×10-7 C6H15O3NH+ 1.7×10-8 7.77

C5H5N 1.5×10-9 C5H5NH+ 6.7×10-6 5.17

How many grams of the bromide salt of the conjugate acid must be combined with how many grams of the weak base, to produce 1.00 L of a buffer that is 1.00 M in the weak base?

grams bromide salt of conjugate acid = _______

grams weak base = _________

Explanation / Answer

from the given, suitable base/conjugate acid : C6H15O3N/C6H15O3NH+

pH of weak base = pka - log(conjugateacid/base)

      pka of C6H15O3NH+ = 7.77

   6.95 = 7.77 - log(x)

x = conjugateacid/base = 6.6

total buffer = 1*1 = 1 mol

total buffer = conjugateacid + base = 1 mol

base= x

conjugateacid = 6.6*x

6.6*x + x = 1

x = 0.1315 mol

base = x = 0.1315 mol

amount of base = 0.1315*149.2 = 19.62 g

No of mol of conjugateacid = 1-0.1315 = 0.8685 mol

amount of conjugateacid = 0.8685*230.2 = 200 g

grams bromide salt of conjugate acid = 200 g

grams weak base = 19.62 g

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