Chemistry S & P Block Elements BUMHS 2022
PMDC Verified Question 58 of 104
Which of the following hydroxide of group I is less soluble in water?
A
NaOH
B
LiOH
C
KOH
D
RbOH
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: LiOH
Concept:

The solubility of an ionic compound depends on the balance between its lattice energy (which favors a solid crystal) and its hydration energy (which favors dissolving).

Solution:

  • Lithium (\( \text{Li}^+ \)) is the smallest cation in Group I.
  • Because of its tiny size, the electrostatic attraction between \( \text{Li}^+ \) and the small hydroxide ion (\( \text{OH}^- \)) is exceptionally strong.
  • This results in a very high lattice energy for Lithium hydroxide (\( \text{LiOH} \)).
  • Because it takes so much energy to break this crystal lattice apart, \( \text{LiOH} \) is the least soluble among the alkali metal hydroxides (solubility of Group I hydroxides generally increases down the group).


Why other options are incorrect:

  • As the cation gets larger (Na, K, Rb), the lattice energy drops significantly faster than hydration energy, making NaOH, KOH, and RbOH highly soluble in water.

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