is a diatomic molecule so it's formula is H₂ (atomic mass of 2 grams/mole),
argon is a noble gas so it's not bonded to any other atom and it's formula is
Ar (atomic mass of 18 grams/mole), and carbon dioxide has the formula CO₂
(atomic mass of 44 grams/mole).
Further recall that we can assume that all are ideal gases since all are pure
All ideal gases follow:
P is the pressure
V is the volume
n is the number of moles
R is the gas constant
T is the temperature in Kelvin
Now notice that P, V, T and R are all constant and equal for all 3 gases making
the n for all 3 gases equal.
Since CO₂ weighs the most per mole it will be the heaviest of all 3 gases,
since H₂ weighs the least per mole it will be the lightest of all 3 gases.