Answer:
5.57 grams
Explanations:
Given the reaction between sodium and oxygen expressed as:

Given the following parameters
Mass of sodium = 16.0g
Determine the moles of sodium

According to stoichiometry, 4 moles of sodium reacts with 1 mole of oxygen, the moles of oxygen that reacted is given as;

Determine the mass of oxygen that reacted

Hence the mass of oxygen that reacted is 5.57 grams