a
Mohr's salt : \(\left({NH}_{4}\right)_{2} {Fe}\left({SO}_{4}\right)_{2} \cdot 6 {H}_{2} {O}\)

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 \(=\frac{6}{12}\)

\(=\frac{1}{2}\)

\(=0.5\)

\(=5 \times 10^{-1}\)