Question -
A solution containing 0.2563 g of naphthalene (molecular mass = 128) in 50 g of carbon tetrachloride yields a boiling point elevation of 0.201∘C while a solution of 0.6216 g of an unknown solute in the same mass of the solvent gives a boiling point elevation of 0.647∘C. Find the molecular mass of unknown solute.
Options:
A .  96.44
B .  86.44
C .  90
D .  88
Answer: Option A : A We know that Kb=ΔTb×W1×Mw21000×W2 First let’s calculate value of KbforCCl4. For CCl4; Kb=0.201×50×1281000×0.2563=5.019kkgmol−1 Now, for calculating mass of unknown solute, we can use the same value of Kb. Mw2=1000×Kb×W2ΔTb×W1 1000×5.019×0.62160.647×50 =96.44
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