calculate how many moles of the product form when 2.57 mol of Na completely reacts.

Question

Assume that there is more than enough of the other reactant.
2Na(s)+O2(g)→Na2O2(s)

Answer 1

The coefficients tell you that.
2.57 molx Na x (1 mol Na2O2/2 mols Na) = ?
Note how mols Na in the numerator cancels with the mols Na in the denominator. That leaves mols Na2O2 in the numerator as the only unit left; you have converted mols Na to mols Na2O2.