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Calculate the freezing point of solution when 1.9 g of MgCl2 (M = 95 g mol−1) was dissolved in 50 g of water, assuming MgCl2 undergoes complete ionization. - Chemistry

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Calculate the freezing point of solution when 1.9 g of MgCl2 (M = 95 g mol−1) was dissolved in 50 g of water, assuming MgCl2 undergoes complete ionization.

(Kf for water = 1.86 K kg mol−1)

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Solution

Given:

Kf = 1.86 K kg mol1

Mass of solute = 1.9 g

Mass of solvent = 50 g

Therefore,

`"Molality of the solution, "m=1.9/95xx1000/50=0.4m`

Also, MgCl2 undergoes complete ionisation and thereby yielding 3 moles of constituent ions for every mole of MgCl2.

i = 3

Now, depression in freezing point is given as

Tf = iKfm

      = 3×1.86×0.4

      = 2.232 K

T273.152.232

    =270.918 K

Hence, the new freezing point of the solution is 270.92 K.

Concept: Colligative Properties and Determination of Molar Mass - Depression of Freezing Point
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