How many molecule are present in one `mL` of water vapour of `STP` ?

Explanation

`22.4` litre water vapour at `STP` has `= 6.023 xx 10^(23)` molecules
`:. 1 xx 10^(-3)` litre water vapours at `STP` has `= (0.693 xx 10^(23) xx 10^(-3))/(22.4) = 2.69 xx 10^(19)` molecules