Design a buffer that has a pH of 5.70 using one of the weak base/conjugate acid
ID: 950221 • Letter: D
Question
Design a buffer that has a pH of 5.70 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 chloride 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 chloride salt of conjugate acid = ______
grams weak base = ______
Explanation / Answer
pOH = 14-5.7 = 8.3
pOH = pkb + log(salt/base)
pkb = -log(1.5*10^(-9)) = 8.82
8.3 = 8.82+logx
x =(salt/acid) = 0.302
so that total buffer (salt+acid) = 1 mol
No of mol of acid = 1/1.302 = 0.77 mol
No of mol of salt = 1-0.77 = 0.23 mol
mass of acid required = 0.77*149.1882 = 114.9 grams
mass of salt = 0.23*(149.1882+36.5) = 42.71 grams
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