The number of water molecules in Mohr's salt $=6$
Potash alum : ${KAl}\left({SO}_{4}\right)_{2} \cdot 12 {H}_{2} {O}$
The number of water molecules in potash alum $=12$
So ratio of number of water molecules in Mohr's salt and potash alum $=\frac{6}{12}$
$=\frac{1}{2}$
$=0.5$
$=5 \times 10^{-1}$