2. Which is the volume of 3.20 moles of O₂?
We are told that at STP one mol of any gas occupies 22.4 L. We will use that relationship to find the volume occupied by 3.20 moles of O₂.
1 mol of O₂ = 22.4 L
volume of O₂ = 3.20 moles of O₂ * 22.4 L/(1 mol of O₂)
volume of O₂ = 71.7 L
Answer: 3.20 moles of O₂ occupies 71.7 L
7. Which is the volume of 100. g of O₂?
In this case we will use the same relationship, but we are given grams instead of moles. So first we have to convert the mass in grams into moles. To do that we use the molar mass of O₂.
atomic mass of O = 16.00 amu
molar mass of O₂ = 2 * 16.00
molar mass of O₂ = 32.00 g/mol
Once that we know that the mass of 1 mol of O₂ is 32.00 g, we can find the number of moles that we have in 100. g of it.
moles of O₂ = mass of O₂ /(molar mass of O₂)
moles of O₂ = 100. g/(32.00 g/mol)
moles of O₂ = 3.13 moles
Finally we can find the volume that 3.13 moles of O₂ occupies (remember that 1 mol of any gas occupies 22.4 L at STP).
volume of O₂ = 3.13 moles * 22.4 L/(1 mol)
volume of O₂ = 70.1 L
Answer: 100. g of O₂ occupies 70.1 L