The molar mass of sodium chlorate is 106 g/mol.
Number of moles of sodium chlorate =212g106g/mol=2 mol
The decomposition reaction of sodium chlorate is as shown.
2NaClO3→2NaCl+3O2
The decomposition of 2 moles of sodium chlorate gives 3 moles of oxygen.
1 mole of oxygen at STP occupies a volume of 22.4 L.
3 moles of oxygen at STP will occupy a volume of 3×22.4=67.2 L.