At 4.00 L, an expandable vessel contains 0.864 mol of oxygen gas. How many liters of oxygen gas must be added at constant temperature and pressure if you need a total of 1.96 mol of oxygen gas in the vessel?

5.07L

Explanation:

The equation to use is Avogadro's Law or  

So when you substitute your values in you end up with V2= 9.074L

To find the change you would subtract 4 to see how much you need to add which is going to be 5.07L