Question

I am working on a chemistry worksheet on buffers with the following information:

Substance 1: HF: type of compound=acid, strength=weak

Substance 2: NaF: type of compound=salt, strength=strong

A solution of the two substances above will form a good buffer solution

My question is how do you determine if a salt is strong or weak? One page of my instructors notes says "the weaker the acid, the stronger the conjugate base" while another page of my notes says "The conjugate of a weak acid acts as a weak base". Please help me understand.

Answer 1

Substance 1: HF: type of compound = acid, strength=weak

Substance 2: NaF: type of compound = salt, strength = strong

A solution of the two substances above will form a good buffer solution

here HF is weak acid and NaOH is strong base.

HF   +   NaOH    -----------> NaF +   H2O

the salt of strong base and weak acid is basic. it has pH > 7 .

here HF is weak acid and conjugate base of HF is F- . that is stronger base.

the weaker the acid, the stronger the conjugate base"

"The conjugate of a weak acid acts as a weak base".

the two statements are correct .

the conjugate base of weak acid is stronger. as compare with other conjugate bases.

but when compared to strong bases like NaOH , KOH , the conjugate base coming from weak acid is always weak base.

for example : of we take HF and HCN

here HF is strong acid than HCN.

now the conjugate base of CN- is stronger than F- . that is comparision with other conjugate acids.