From the balanced equation, 4 moles of Na react to produce 2 moles of Na$_2$O. The mole ratio is 4:2 or 2:1.
If 2.55 mol of Na reacts, the moles of Na$_2$O produced are:
$$ \text{moles of } Na_2O = 2.55 \text{ mol Na} \times \frac{2 \text{ mol } Na_2O}{4 \text{
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