Final answer:
Joe's balloon has 0.2 moles of oxygen, obtained by converting 200 millimoles to moles.
Step-by-step explanation:
To calculate the number of moles in the balloon, you need to know the molar mass of oxygen. The molar mass of oxygen is approximately 32 grams per mole.
To find out how many moles are in the balloon, we need to convert millimoles to moles. Since one mole is equal to 1000 millimoles, you can calculate the number of moles in the balloon by dividing the number of millimoles by 1000.
So, in this case:
200 millimoles ÷ 1000 = 0.2 moles
Therefore, there are 0.2 moles of oxygen in Joe's balloon.