Question

Molar mass and density?

Dry air (0% humidity) is approximately 78% N2, 21% O2, and 1% Ar by moles. Calculate the molar mass and density of dry air at 0 °C and 1.0 atm. Assume ideal behavior and use two significant figures in your answers.

Answer 1

Molar masses of constituent elements and their corresponding number of moles in air

`"Element"" ""molar mass"" "" "" "mol%`

`" "N_2" "" "28.013" g/mol"" "" "78%`

`" "O_2" "" "31.998" g/mol"" "" "21%`

`" "Ar" "" "39.948" g/mol"" "" "01%`

Taking weighted average the molar mass of air is

`M_"air"->(78xx28.013+21xx31.998+1xx39.948)/100~~28.969" g/mol"`

At STP Molar volume is `22.4"mol/L"`

`"So density of air at STP "="molar mss"/"molar volume"`

`=(28.969g/"mol")/(22.4L/"mol")~~1.293g/L`