Design a buffer that has a pH of 10.01 using one of the weak base/conjugate acid
ID: 1013661 • Letter: D
Question
Design a buffer that has a pH of 10.01 using one of the weak base/conjugate acid systems shown below.
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 produce1.00 L of a buffer that is 1.00 M in the weak base?
grams bromide salt of conjugate acid = _________
grams weak base = ___________
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
Explanation / Answer
pH of BASIC buffer = 14 - ( pkB + log(acid/base))
pkb ofCH3NH2 = -log(4.2*10^-4) = 3.37
10.01 = 14-(3.37+logx)
X = (acid/base) = 4.17
TOTAL NO OF MOL OF BUFFER(acid+base) = 1*1 = 1 mol
No of mol of conjugate acid required = 1-0.193 = 0.807 mol
No of mol of base required = 1/5.17 = 0.193 mol
mass of weak base(CH3NH2) required = 0.193*31.06 = 6 grams
mass of bromide salt of conjugate acid(CH3NH3Br) required = 0.807*112 = 90.384 grams
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