12. According to Brönsted-Lowry theory,which of the following substance is an acid only? (a) HS- (b) CH3COO- (c) NH4+ (d) HCO3-?

A Brönsted-Lowry acid is a proton donor, while a Brönsted-Lowry base is a proton acceptor.

(A) can be both an acid and a base.
HS(-) ⇌ H(+) + S(2-) ...... HS(-) acts as an acid.
HS(-) + H(+) ⇌ H2S ...... HS(-) acts as a base.

(B) can be a base only.
CH3COO(-) + H(+) ⇌ CH3COOH ...... CH3COO(-) acts as a base.

(C) can be an acid only.
NH4(+) ⇌ NH3 + H(+) ...... NH(+) acts as an acid.

(D) can be both an acid and a base.
HCO3(-) ⇌ CO3(2-) + H(+) ...... HCO3(-) acts as an acid.
HCO3(-) + H(+) ⇌ H2CO3 ...... HCO3(-) acts as a base.

...... The answer is : (C) NH4^+