Calculate the mass of compound (molar mass-256 g/mol) to be dissolved in 75 g of benzene to lower its freezingpoint by 0.48 K. (Kf-5.12 kg/mol)
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Answer:
Let W g of compound is to be dissolved.
Number of moles of compound, n=
molar mass
mass
=
256
W
Mass of benzene = 75 g =
1000
75
kg=0.075kg
Molality of solution, m=
mass of solvent in kg
number of moles of solute
m=
256×0.075
W
=
19.2
W
m
The depression in the freezing point, ΔT
f
=0.48K
The molal depression in freezing point constant =K
f
=5.12Kkg/mol
ΔT
f
=K
f
m
0.48=5.12×
19.2
W
W=
5.12
0.48×19.2
=1.8
Hence, 1.8 g of solute is to be dissolved.
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