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

The correct option is B $CsF>RbF>KF>NaF>LiF$

$LiI>NaI>KI>RbI>Cal$ (solubility)
Lattice energy $\frac{1}{r_{+}+r_{-}}$
Since, $r_{-}>r_{+}$ the sum will not change too much as $r_{+}$ increases. As a result, the lattice energy will not change significantly as $r_{+}$ increases. Thus decreases in lattice energy is not as fast as decreases in hydration energy. Thus, more decreases in hydration energy result in a decrease in solubility. Thus, solubility decreases as
$LiI>NaI>KI>RbI>Cal$
With a small anion as $\left(F^{-}\right)$ the lattice energy decreases more rapidly along a series of salts with increasing cation size and thus solubility increases.
$LiF<NaF<KF<RbF<CsF$