Which of the following aqueous solution has the highest freezing point ?

Options

A. 1% (w/w) glucose
B. 1% (w/w) NaCl
C. 1% (w/w) sucrose
D. 1% (w/w) `CaCl_(2)`

Answer

B

Explanation

Assuming complete dissociation of electrolytes :
Molal conc. of 1% glucose `=(1)/(180)xx(1000)/(100)=1/18`
`DeltaT_(b)=iK_(b)xxm=1xxK_(b)xx (1)/(18)`
`=(K_(b))/(18)=0.055K_(b)`
Molar conc. of 1% NaCl solution
`=(1)/(58.5)xx(1000)/(100)=(1)/(5.85)`
`DeltaT_(b)=iK_(b)xxm=2xxK_(b)xx(1)/(5.85)`
`=0.34K_(b)`
Molar conc. of 1% sucrose
`=(1)/(342)xx(1000)/(100)=(10)/(342)`
`DeltaT_(b)=iK_(b)xxm=1xx(10)/(342)xxK_(b)`
`=0.029K_(b)`
Molar conc. of 1% `CaCl_(2)=(1)/(111)xx(1000)/(100)=(10)/(111)`
`DeltaT_(b)=iK_(b)m=3xxK_(b)xx(10)/(111)=0.27K_(b)`
`:.DeltaT_(b)` is maximum for 1% NaCl solution and hence it will have highest boiling point.