In this question, we have to find the density of a gas, and we can use the Ideal gas Law to find it, but we need to make a few changes to the formula, the initial formula is:
PV = nRT
Where:
P = pressure
V = volume
n = number of moles
R = gas constant
T = temperature
But as we can see, there are some missing informations in order to use this formula, and density is mass/volume, so we need to use the ideal gas formula to find mass/volume
One thing we can do is to convert n (number of moles) into mass/molar mass, which is exactly what number moles means, therefore the formula would be:
PV = m/MM * R * T
Now we have mass, and volume, but they are separated in the formula, we need to bring these two together, let's do it mathematically:
PV = m/MM * R * T
P * MM/R * T = m/V, here we have mass/volume, which is exactly density, now we use the informations available to finally find the answer
For this formula we have:
P = 3 atm
MM = for O2 is 32g/mol
R = gas constant is 0.082
T = 400 K
m/V = 3 * 32/0.082 * 400
m/V = 96/32.8
m/V = 2.93 g/L