Two moles of a nonelectrolyte solute are dissolved in 1 kg of an unknown solvent. The solution freezes at 7.8 degrees Celsius below its normal freezing point. What is the molal freezing point constant of the unknown solvent? What is your prediction for the identity of the solvent?
The Givens are :
mol(solute)=2mol
Nonelectrolyte (no. of ion=1)
mass(solvent)=1kg
tf(solution)=7.8c
The Unknown is :
Kf=?
Because it says make the solution freez lower than normal freezing point (which is 0c) so the answer should be in negative, right ??
m = moles/kg solvent
m = 2/1 = 2m
delta T = Kf*m
7.8 = Kf*2
Kf = 7.8/2 = ??
Here is a list of constants. Pick yours.
http://en.wikipedia.org/wiki/Freezing-point_depression
-3.9
To find the molal freezing point constant of the unknown solvent, we can use the equation:
ΔTf = Kf * m
Where:
ΔTf is the freezing point depression (given as 7.8 degrees Celsius in this case)
Kf is the molal freezing point constant (what we want to find)
m is the molality of the solution, which is the number of moles of solute per kilogram of solvent.
In this case, we are given that two moles of solute are dissolved in 1 kg of solvent. So the molality (m) can be calculated as:
m = (moles of solute) / (mass of solvent in kg)
m = 2 moles / 1 kg
m = 2 mol/kg
Now, we can substitute the given values into the equation and solve for Kf:
7.8 = Kf * 2
Dividing both sides by 2, we get:
Kf = 7.8 / 2
Kf = 3.9 °C·kg/mol
Now, to predict the identity of the solvent, we can compare the calculated Kf value to the known molal freezing point constants of common solvents. The molal freezing point constant is specific to each solvent.
Based on the value of Kf = 3.9 °C·kg/mol, we can predict that the solvent is likely water (H2O). The molal freezing point constant for water is approximately 1.86 °C·kg/mol.
Hence, based on the comparison, we can conclude that the unknown solvent is likely water.