$\begin{array}{1 1}(a)\;Na_2SO_4.10H_2 O&(b)\;Na_2SO_4\\(c)\;NaSO_49H_2O&(d)\;Na_2SO_3 10H_2O\end{array}$

The empirical formula $=Na_2SH_{20}O_{14}$

Empirical formula mass =$(2\times 23)+32+(20\times 1)$

$\qquad\qquad\qquad\qquad\;=322$

Molecular mass =322

$n=\large\frac{Mol.mass}{Emp.mass}$

$\;\;\;=\large\frac{322}{322}$

$\;\;\;=1$

$\therefore$ Molecular formula =$Na_2SH_{20}O_{14}$

Whole of the hydrogen is present in the form of water.Thus,10 water molecules are present in the molecule.

So molecular formula =$Na_2SO_4.10H_2 O$

Hence (a) is the correct answer.

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