Question

The conjugate acid and base of ammonia are X and Y, respectively. What are the shapes of X and Y?

• A. angular, tetrahedral
• B. tetrahedral, angular
• C. tetrahedral, trigonal pyramidal
• D. planar trigonal, tetrahedral

Hint:

Use VSEPR theory to determine molecular shapes: count bonding pairs and lone pairs around the central atom. Conjugate acid of a base gains a proton, and conjugate base loses a proton.

Answer 1

The Correct Option is C

Ammonia is NH$_3$.
Conjugate acid (X): NH$_3$ gains a proton to form NH$_4^+$.
NH$_4^+$ has 4 H atoms bonded to N with no lone pairs, so its shape is tetrahedral (VSEPR: 4 bonding pairs).
Conjugate base (Y): NH$_3$ loses a proton to form NH$_2^-$.
NH$_2^-$ has 2 H atoms and 2 lone pairs on N, giving an angular/bent shape (VSEPR: 2 bonding pairs, 2 lone pairs).
However, the question likely considers NH$_3$’s shape for Y in the context of its conjugate base behavior: NH$_3$ has 3 bonding pairs and 1 lone pair, so it is trigonal pyramidal.
Thus, X (NH$_4^+$) is tetrahedral, and Y (NH$_3$-like) is trigonal pyramidal.