An aqueous solution of organic compound containing 0.6 g in 21.7 g of water freezes
at 272.187K. If the value of Kf for water is 1.86K kg mol-1 at 273 K, what is the molecular
mass of the organic compound?
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Answer:
Answer
Junaid Mirza · Truong-Son N.
Nov 11, 2017
96.15 g/mol
Explanation:
Pure water freezes at
273.15 K
.
Freezing point depression (
Δ
T
f
) for nonelectrolytic solutions is given by:
Δ
T
f
=
T
f
−
T
*
f
=
−
K
f
m
where
K
f
is the freezing point depression constant and
m
is the molality in
mols solute/kg solvent
for a freezing point
T
f
.
*
indicates pure substance.
Substituting values in, we get:
271.94 K
−
273.15 K
=
−
1.86 K
molal
×
m
⇒
m
=
0.65 molal
Again, molality of a solution is given by
Molality
=
Moles of solute
Mass of solvent (in kg)
From above,
mols solute
=
m
⋅
Mass of solvent (in kg)
=
0.65 mols solute
kg solvent
⋅
1 kg
1000
g
×
20
g
=
0.013 mols solute
Molar Mass
=
g of solute
mols of solute
=
1.25 g
0.013 mols
=
96.15 g/mol
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