Suppose that a student calculated Kf of biphenyl to be 81.2C kg/mol. calculate the mass of an unknown if a solution containing 1.64 g of unknown in 18.22 g of biphenyl froze at 39.9 C
Read your question and rephrase it to take care of the errors. You surely don't mean calculate mass. The mass is given. And you don't give the normal freezing point so we don't know delta T. And that 81.2 kg/mol? Kf is usually given in C/m.
Its asking for the molar mass of the unknown, sorry.
I don;t know what 81.2 kg/mol means for a Kf. I don't know the normal freezing point of biphenyl.
Here is how you do it if you find that information.
delta T = (39.9-normal f.p.) = Kf*m
Substitute and solve for m = molality
m = mols solute/kg solvent
Substitute and solve for mols solute.
mols solute = grams solute/molar mass solute. You know mols and grams, solve for molar mass.
But you need the normal freezing point and a viable Kf.
183.696 grams
To solve this problem, you'll need to use the freezing point depression equation:
ΔT = Kf * m
Where:
ΔT is the change in freezing point temperature
Kf is the molal freezing point depression constant
m is the molality of the solution
Step 1: Calculate the molality of the unknown compound.
Molality (m) is defined as the number of moles of solute per kilogram of solvent.
First, convert the mass of biphenyl from grams to kilograms:
mass = 18.22 g biphenyl = 0.01822 kg biphenyl
Next, calculate the moles of biphenyl:
moles biphenyl = mass biphenyl / molar mass biphenyl
Since the molar mass of biphenyl (C12H10) is 154.21 g/mol:
moles biphenyl = 0.01822 kg biphenyl / 154.21 g/mol biphenyl = 0.000118 mol biphenyl
Now, we have the moles of biphenyl present in the solution.
Using the equation:
molality (m) = moles solute / mass of solvent (in kg)
We can calculate the molality:
m = moles unknown compound / mass biphenyl
Since we know the mass of the unknown compound is 1.64 g, convert it to moles:
moles unknown compound = mass unknown / molar mass unknown
Unfortunately, information about the molecular formula or molar mass of the unknown compound is not provided in the question. Therefore, we cannot proceed further without that information.