Question

A solution containing 7g of a non-volatile solute in 250g of water boils at 373.26K. Find the molecular mass of solute?

Answer 1

Given,

`DeltaT = 100.11°"C" - 100.0°"C" = 0.11°"C"`

`m_(H_2O) = 0.250"kg"`

`K_"b" = (0.512°"C")/m`

Recall,

`DeltaT_"b" = iK_"b"m`

Since our solute is non-volatile, we assume `i approx 1`.

Hence,

`=> n/m_(H_2O) = (DeltaT_"b")/(iK_b)`

`=> n = (DeltaT_"b" * m_(H_2O))/(iK_b) approx 5.37*10^-2"mol"`, and by extension,

`5.37*10^-2"mol" * ("MW"_"solute")/"mol" = 7"g"`

`=> "MW"_"solute" = (7"g")/(5.37*10^-2"mol") approx (130"g")/"mol"`

is the molecular weight of the solute given your data.