A 1 L flask cantaing vapours of methyl alcohol (molar mass 32) at a pressure was `10^(-3)` mm. How many molecules of mehtyl alcohol are left in the flask ?

Explanation

`P_(1)=10^(-3)mm`
`V_(1)=1000cm^(3)`
`T_(1)=298K`
For converting this volume to the volume at STP, apply
`(P_(1)V_(1))/(T_(1))=(P_(2)V_(2))/(T_(2))`
i.e., `(10^(-3)xx1000)/(298)=(760xxV_(2))/(273)`
or `V_(2)=1.205xx10^(-3)cm^(3)`
Now `22400cm^(3)` at STP `=6.02xx10^(23)` molecule
`therefore1.205xx10^(-3)cm^(3)` at STP
`=(6.02xx10^(23))/(22400)xx1.205xx10^(-3)` molecules
`=3.24xx10^(16)` molecules