Question

The solubility of fluorides of alkali metals in water is:
A. LiF > NaF > KF > RbF > CsF
B. CsF > RbF > KF > NaF > LiF
C. LiF > KF > NaF > CsF > RbF
D. CsF > KF > NaF > RbF > LiF

Answer 1

Hint: Solubility is the property of a solid, liquid, or gaseous chemical substance called a solid, liquid, or gaseous solution dissolving solute. Its tendency to dissolve in a solution with a given substance, the solute. It is calculated at equilibrium in terms of the overall volume of solute dissolved in a solvent. The solution which results is called a saturated solution.

Complete Step by Step Solution: To find solubility of fluorides of alkali metals in water we can find this with the help of Fazan’s rule:
Before we start talking about the Fazan’s rule let's discuss the two terms which we used in the rule.
Polarizing power: it is the power of cation by which it pulls the electron density of anion towards itself. More the polarizing power more is the covalent character.
Polarizability: it is the tendency of anion to lose its electron density. Greater polarizability is the covalent character.
Fazan’s rule:
Size of cation: smaller the size of cation more is the polarizing power, more is the covalent character thus the solubility decreases.
Charge of cation: More the charge on cation more is the polarizing power, more is the covalent character thus the solubility decreases.
Size of anion: More the size of anion more is the polarizability, more is the covalent character thus the solubility decreases.
In the case of the s block element, we will see the difference in size of cation and anion i.e. solubility is directly proportional to difference in size. As the difference in size increases solubility increases.
As here, we are talking about alkali metal fluorides i.e. LiF, KF, NaF, CsF, RbF.
So, here F is fixed (fluorine is so small so we have to choose cation for higher solubility) then we will see the size of cation, on going down the group size of cation increases so the difference also increases so solubility increases.

Hence, option B is correct.

Note:
- Always remember, smaller the size of cation more is the polarizing power, more is the covalent character thus the solubility decreases.
- More the charge on cation more is the polarizing power, more is the covalent character thus the solubility decreases
- More the size of anion more is the polarizability, more is the covalent character thus the solubility decreases.