Stoichiometry question
One thing I see:  when you work out the MW (or AW) of X, you assume 4 moles of X based on the chemical formula.  But, properly, you have twice as many moles of X as Na ($\ce{Na_2X_4}$), and you found just 0.5 mole of Na.  So instead of 4 moles of X you have just one mole of X making the atomic weight if X 10.8 g/mol not 2.7.  Hmmm, isn't there an element in the second row of the Periodic Table with that atomic weight?