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Non-Ideal Solutions

Calculate the...

Question

Calculate the normal freezing point of a sample of sea water containing 3.8 % nacl and 0.12 % of mgcl2 by mass given kf = 1.86 K kg/ mol molar mass of nacl 58.5 and mgcl2 95 g/mol

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Solution

Let the mass of solution be 100 g
Mass of NaCl =3.8 g
Mass of MgCl2=0.12 g
Mass of water =100−3.8−0.12=96.08 g
No. of moles of NaCl =mass/molar mass
=3.8/58.5=0.0649 moles
No. of moles of MgCl2 =0.12/95.2
=0.0012 moles
Total no. of moles of solute =0.0649 +0.0012 =0.0661 moles
Molality of solution = Moles of solute/ kg of solvent
=0.0661/0.0960=0.6885 m
ΔTf =Kbm
where ΔTf is depression in freezing point
ΔTf = 1.86 × 0.6885 =1.2806 K
ThusNormal freezing point of sea water = 273−1.2806 =271.71 K

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