On dissolving `0.25 g`of a non-volatile substance in`30 mL` benzene `(density 0.8 g mL^(-1))`, its freezing point decreases by `0.25^(@)C` . Calculate the molecular mass of non-volatile substance `(K_(f) = 5.1 K kg mol^(-1))`.

Explanation

Mass of benzene, W = volume `xx` density
`=30 xx 0.8 = 24g`
Given, `K_(f)=5.12 "K kg mol"^(-1),w=0.25g,Delta T=0.40^(@)C`
We know that,
`m=(1000K_(f)xxw)/(WxxDeltaT)`
`= (1000xx5.12xx0.25)/(24xx0.40)=133.33`.