Question

For a dilute solution containing 2.5g of a non-volatile non-electrolyte solute in 100g of water, the elevation in boiling point at 1 atm pressure is `2^(@)"C"`. Assuming concentration of solute is much lower than the concentration of solvent, the vapour pressure (mm of Hg) of the solution is (take `K_(b)`=0.76 K kg `"mol"^(-1)`)
A. 724
B. 740
C. 736
D. 718

Answer 1

Correct Answer - A
`DeltaT=K_(b)xxm`
`2=0.76xxm`
`m=(2)/(0.76)=2.63`
Mole fraction of solute can be calculated as
`m=(x_(B)xx1000)/((1-x_(B))m_(A))`
`2.63=(x_(B)xx1000)/((1-x_(B))xx18)`
`x_(B)=0.045`
`(p_(0)-p)/(p_(0))=x_(B)`
`(760-p)/(760)=0.045`
or `p=725.8` mm
It is close to 724 mm]