Question

Why does 1 mol of sodium chloride depress the freezing point of 1 kg of water almost twice as much as 1 mol of glycerin?

Answer 1

This can be explained using colligative property shown by solution
When a non volatile solute dissolved in a solvent[for example water] the
freezing point of solvent will decrease .This depression in freezing point
depends on number of solute particles in the solution .It is given by the
formula
dTf = i*Kf*m
where Kf= is the depression constant for solvent, m= molality and i=vant Hoff's
factor ,which is a measure of number of solute particles in solution
We know molality = number of moles of solute/mass of solvent .
For the given two solutions
molality = 1 m also
Kf will be same
But NaCl is an ionic salt hence it will ionize in water to 2 ions
NaCl--> Na+(aq) +Cl-(aq)
thus vant Hoff's factor for NaCl ,i= 2

where as glycerin is a non electrolyte which will not ionize in water. Hence
its vant hoff's factor ,i= 1

depression in F.P ,dTf= i.Kf.M
Hence for NaCL dTf will be almost twice than that of glycerin
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.. hope it is helpful