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

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Answer 1 · 2018-06-23T18:58:02

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.