Calculate the mass of a non-volatile solute ( molar mass 40g `mol^-1`) which should be dissolved in 114g octane to reduce its vapour pressure to 80%.

Explanation

We are given that
Molecular mass of solute `(M_(B))=40`
Mass of octane `(w_(A))=114g`
Molecular mass of octane `(M_(A))=114`
Lowering in vapour pressure can be calculated as `(p^(@)-p_(s))/(p^(@))=(100-80)/(100)=0.2`
Here,
`p^(@)` is the pressure of the pure solvent.
`p_(s)` is the pressure of the solution.
We are to find the mass of non-volatile solute `(w_(B))`
Using the relation
`M_(B)=(w_(B)xxM_(A))/(w_(A)xx((p^(@)-p_(s))/(p^(@))))orw_(B)=M_(B)xxw_(B)xx((p^(@)-p_(s))/(p^(@)))/(m_(A))=(40xx114xx0.2)/(114)=8g`
`therefore` Mass of the non-volatile solute is 8 g.