Aballoon is first filled with 4.00 moles of hydrogen. then, 2.00 moles of oxygen are added. if the volume was initially 21 l when just the hydrogen was present, what is the volume after the oxygen is added?

Initially the balloon was 21 liters with 4.00 moles of hydrogen
Therefore 1 mole of hydrogen had 21/4 = 5.25 liters
Therefore; 1 mole of oxygen would still contain 5.25 liters
Hence; 2 moles of oxygen contain 5.25 × 2 = 10.5 liters
Thus; the total volume of the balloon will be;
  21 +10.5 = 31.5 liters

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