Calculate the number of moles of sodium oxide that should be produced with 6.50 g of sodium react with oxygen gas

First of all write a balanced equation.

4Na + O2 - > 2Na2O

You are starting with a mass of sodium. You need to use sodium's molar mass. Take this quantity times mole to mole ratio between sodium and sodium oxide to get moles of sodium oxide produced.

.141 moles of sodium oxide produced