Question

Design a buffer that has a p of 11.10 using one of the weak base acid systems shown below. How many grams of the chloride salt of the conjugate acid 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 acid grams weak base

Answer 1

pH = 11.10

pOH = 14-11.1 = 2.90

pKb nearest to this is CH3NH2 so:

pOH = pKB + log(BH+/B)

pKB = -log(4.2*10^-4) = 3.3767

2.90 = 3.3767 + log(CH3NH3Cl / CH3NH2)

CH3NH3Cl / CH3NH2 = 10^(2.90-3.3767 ) = 0.333656

and we know

CH3NH3Cl +CH3NH2 = 1

solve

CH3NH2 = 1-CH3NH3Cl

CH3NH3Cl / (1-CH3NH3Cl)= 0.333656

CH3NH3Cl = 1 - 0.333656*CH3NH3Cl

1.333656*CH3NH3Cl = 1

CH3NH3Cl = 1/1.333656 = 0.74981

so

CH3NH2 = 1-CH3NH3Cl

CH3NH2 = 1-0.74981 =0.25019

so

mass CH3NH2   = mol*MW = 0.25019*31.0571 = 7.7701 g

mass ofCH3NH3Cl = mol*MW = 0.74981*67.5180 = 50.625 g