What mass of `NaCI ("molar mass" =58.5g mol^(-1))` be dissolved in `65g` of water to tower the freezing point by `7.5^(@)C`? The freezing point depression constant, `K_(f)`, for water is `1.86 K kg mol^(-1)`. Assume van't Hoff factor for `NaCI` is `1.87`.

Answer

8.199g

Explanation

`DeltaT_(f)=iK_(f)m=iK_(f)(w_(2))/(M_(2))xx(1)/(w_(1))xx1000," "7.5=1.87xx1.86xx(w_(2))/(58.5)xx(1)/(65)xx1000 or w_(2)=8.199g.`