Question

Arrange the following gases in order of decreasing density at STP. Ne, O2, F2, Cl3

Answer 1

Answer: Cl₃ > F₂ > O₂ > Ne

Explanation:

1) All the gases are at the same temperature and pressure: STP (O°, 1 atm).

2) All the gases are represented by one molecule (or mole of molecules, it is the same for our puroposes).

3) As per Avogadro's law and the ideal gas equation, the same number of particles (of any gas) occupy the same volume at the same temperature and pressure.

For example, at STP the volume of 1 mol of gas is 22.4 liter.

4) Since, density is equal to mass / volume, and you assume the same number of molecules of each gas, the order of the densities is the same that the order of the molar masses.

5) Then, you just need to state the molar mass of each gas and order in decreasing order of molar mass to have the correct order for density.

6) Molar masses:

i) Ne: 20.180 g/mol

ii) O₂: 2× 15.999 g/mol = 31.998 g/mol

iii) F₂: 2×18.998 g/mol = 37.996 g/mol

iv) Cl₃: 3×35.453 g/mol = 106.359

7) Then the order is: 106.359 > 37.996 > 31.998 > 20.18 ⇒ is Cl₃ > F₂ > O₂ > Ne.

Note: the values calculate are not of densities but of molar masses. The order is what is the same.

Answer 2

The main formula is

D=density at STP = molar mass  / 22.4
dNe = 20/22.4=0.89
dO2= 2*16/22.4=1.42
dF2=2*18/22.4=36/22.4=1.60
dCl3=3*35.5/22.4=4.75


in order of decreasing density at STP means

dNe <dO2 <dF2<dCl3