Using the equation 4Na+O2---->2Na2O 4 moles of Sodium reacts with 1 mole of oxygen.
The moles of sodium in 21.8 gram is 0.949moles .(21.8g/23gmol-1)
Because 4 moles of sodium to one mole of oxygen you divide by 0.949 by 4
The moles of oxygen need to react with 0.949 moles of sodium is 0.0413(0.949/4).