Density difference: Methane and air

Question

Why is methane less dense than air?

I know that CO2 is denser than air,

N02 is denser than air,

so i thought that methane is denser than air, but it turns out that methabe is less dense.

I ask my teacher about it, she said thta air is a mixture thus, it's very dense, but if so, than this thoery cannot support why C02 and N02 is denser.

Answer 1

22.4 litres is called the "Molar volume of a gas"

it is said that every gas has 1 mole of itself in 22.4 litres @ STP

so if you knew the molar mass of a gas, you know its density ...

for you have that mass in 22.4 litres. the larger the molar mass the greater is its density

air is 20% O2 @ 32 g/mol ,... & 80 % N2 @ 28 g/ mol,... at this % , air has an average molar mass of 29 g/mol

Molar masses:

H2 2g/mol

He 4g/mol

CH4 16g/mol

air 29g/mol

CO2 44g/mol

NO2 46 g/mol

SO2 64 g/mol

SO3 96 g/mol

these molar masses show that H2, He, & CH4 are less dense than air, & that CO2 , NO2, SO2 & SO3 are more dense than air, but we will calc it any way ..

since 1 mole of every gas has a volume of 22.4 litres @ STP, the densities are:

H2 2g/22.4L = 0.09 g / litre

He 4g/22.4L = 0.18 g / litre

CH4 16g/22.4L = 0.71 g / litre

air 29g/22.4L = 1.29 g/litre

CO2 44g/22.4L = 1.96 g / litre

NO2 46 g/22.4L = 2.05 g / litre

SO2 64 g/22.4L = 2.86 g / litre

SO3 96 g/22.4L = 4.29 g/ litre

======================================...

I used what is called "a weighted average" to get the average molar mass of air:

@ 20 % O2 :  (20) (32g/mol) = 640

@ 80 % N2 : (80) (28 g/mol) = 2240

air = (640 + 2240) / 100

air = 2880 / 100

air = 28.8 grams / mole on the average

p.s. the densities all change proportionally when temperature & pressure vary from STP,  but what ever was less dense than air will still be, & vice versa