Stoichiometry question I tried solving the following question but am not sure I did so correctly and simply wished to verify it here.

In 50.3 gr of a compound whose formula is $Na_2X_4O_7$ (where X is an unknown atom) are 1.75 moles of oxygen atoms.

a) What is the number of Na atoms in the sample?

I figured the number of moles would simply be 1.75*2/7=0.5, hence number of atoms would be $0.5*N_A$.

b) What is the molar mass of X?

I figured that since there are 1.75 moles of oxygen at 16 gr/mole and 0.5 moles of Na at approx. 23 gr/mole, the MW of oxygen and Na together would be 1.75*16+0.5*23=39.5 gr, hence the MW of X would be: (50.3-39.5)/4=2.7 gr/mole.

I am now told that when the compound comes in contact with 500ml of hot water (in excess), the following reaction takes place:

$Na_2X_4O_{7(s)}+H_2O -&gt;XH_{3(g)}+Na^+_{(aq)}+OH^-_{(aq)}+O_{2(g)}$

Moreover, in a certain process 0.84 moles of $XH_{3(g)}$ were formed.

1) How many grams of gaseous oxygen were formed in the process?

I first balanced the reaction thus:

$Na_2X_4O_{7(s)}+7H_2O -&gt;4XH_{3(g)}+2Na^+_{(aq)}+2OH^-_{(aq)}+6O_{2(g)}$

I figured that if 0.84 moles of $XH_3$ were formed then 0.84*1.5 would be the number of oxygen moles formed, weighing 20.16 gr.

2) How many grams of $Na_2X_4O_{7(s)}$ reacted in the process?

I figured that 0.84/4 moles of $Na_2X_4O_{7(s)}$ must have reacted in the process, weighing (0.21/3.25) * 50.3=3.25 gr.

3) What are the volumes of the oxygen and the $XH_{3(g)}$ formed in the process at room conditions?

I figured $V_{O_2}=n_{O_2}*0.082*298$ and $V_{XH_3}=n_{XH_3}*0.082*298$, where $n_{O_2}=1.26 moles$ and $n_{XH_3}=0.84 moles$

4) What is the concentration of $OH^-$ ions in the solution obtained (500ml)?

I figured the answer would simply be 0.42/0.5=0.84M, as 0.42 moles of $OH^-$ were formed.

I'd appreciate any comments on my attempt above.
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?