A 4 g mixture of  $N{a}_{2}O$  and NaCl was dissolved in 400 mL of 0.3 M HCl. After complete reaction, the resulting solution required 200 mL of 0.2 M NaOH for complete neutralization of the excess HCl. What mass % of  $N{a}_{2}O$  is present in the mixture?