`6.2g` of a sample containing `Na_2CO_3, NaHCO_3` and non-volatile inert impurity on gentle heating loses 5% of its mass due to reaction `2NaHCO_3 rarr Na_2 CO_3 + H_2 O + CO_2` . Residue is dissolved in water and formed `100mL` solution and its `10mL` portion requires `7.5 mL` of `0.2M` aqueous solution of `BaCI_2` for complete precipitation of carbonates. Determine mass (in gram) of `Na_2 CO_3` in the original sample.