Maharashtra BoardEnglish MediumSTD 12 ScienceChemistrySolutions3 Marks
Question
What is a relationship between freezing point depression and concentration of solute (or solution)?
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Answer
It is observed experimentally that as the concentration of a solution increases, the freezing point of the solution decreases and hence the depression in the freezing point $\left(\Delta T_f\right)$ increases.
The depression in the freezing point of a solution is directly proportional to the molal concentration (expressed in $mol _{ kg ^{-1}}$ ) of the solution. Thus, $\Delta T _f \propto m$ where $m$ is the molality of the solution. $\therefore \Delta T _{ f }= Kfm$, where $Kf$ is a constant of proportionality. If $m =1 molal$, $\Delta T_f=K_f$. Hence $K_f$ is called the cryoscopic constant or molal depression constant. $K _{ f }$ is characteristic of the solvent.
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