Question

# Aqueous solutions of one of the following salts is basic, which one and how do you...

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?

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

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