Question

Aqueous solutions of one of the following salts is basic, which one and how do you figure it out?

a) NH4F

b) NH4OCN

c) NH4OBr

d) NH4CH3COO

e) NH4NO2

I understand that NH4 is a weak acid having Ka = 5.68 * 10 ^-10

but i don't know how to determine the rest in order to narrow the list down to the only basic salt. and why are all of the rest not basic salts?

Answer 1

Let us do pH caculation for,

a) NH4F

NH4F ---> NH4+ + F-

both will hydrolyze to give,

NH4+ + H2O <==> NH3 + H3O+

and,

F- + H2O <==> HF + OH-

a quick way to determine pH = 1/2(pKa(NH3) + pKa(HF))

Taking values from lierature,

pH = 1/2(9.25 + 3.15) = 6.20

So solution is acidic

b) NH4OCN

pKa for NH3 and HOCN

pH = 1/2(9.25 + 3.46) = 6.35

Solution is acidic

c) NH4OBr

pKa for NH3 and HOBr

pH = 1/2(9.25 + 8.63) = 8.94

solution is basic

d) NH4CH3COO

pKa for NH3 and CH3COOH

pH = 1/2(9.25 + 4.74) = 6.995

solution acidic

e) NH4NO2

pKa for NH3 and HNO2

pH = 1/2(9.25 + 3.35) = 6.30

solution acidic

Thus looking at the above data for pH we can say the salt which would give a basic solution would be,

c) NH4OBr