Question

Which structure corresponds to the predominant form of the molecule near pH=7?

a) H₃O⁺
b) OH⁻
c) NH 4⁺
​ d) CO ₃²⁻
​

Answer 1

The predominant form of the molecule near pH=7 is NH4+.

Step-by-step explanation:

The predominant form of a molecule at pH=7 depends on whether it acts as an acid or a base. At pH=7, a neutral pH, the molecule that corresponds to the predominant form is the conjugate acid/base pair that is closest to pH=7.

In this case, the molecule NH3 can act as a base and NH4+ can act as its conjugate acid. The pKa of NH4+ is about 9.25, which means that at pH=7, NH4+ will exist in its predominant form.

Therefore, the correct answer is c) NH4+.