Adding 1.00 g of benzene, C6H6, to 80 g of cyclohexane, C6H12, lowers the freezing point of the cyclohexane from 6.5 to 3.3 degree celsius. What is the value of kf for cyclohexane?
Why did the cyclohexane go to the therapist? Because it was feeling a little depressed and needed someone to 'freezing point' out its problems!
But let's get back to your question. The freezing point depression (∆Tf) is given by the formula ∆Tf = Kf * m, where Kf is the molal freezing point depression constant and m is the molality of the solution.
Given that the freezing point of cyclohexane decreased from 6.5 °C to 3.3 °C, the ∆Tf would be 6.5 °C - 3.3 °C = 3.2 °C.
Since you added 1.00 g of benzene to 80 g of cyclohexane, the molality (m) can be calculated as follows:
m = (moles of solute) / (mass of solvent in kg)
To find the moles of benzene, we divide the mass by the molar mass:
moles of benzene = 1.00 g / (78.11 g/mol) = 0.01281 mol
Now, let's calculate the molality:
m = 0.01281 mol / (0.08 kg) = 0.1601 mol/kg
Finally, we can calculate the value of Kf by rearranging the formula:
Kf = ∆Tf / m = 3.2 °C / 0.1601 mol/kg = 19.986 mol/kg
So, the value of Kf for cyclohexane is approximately 19.986 mol/kg. Just remember, it's always important to put a little "mol" effort into chemistry!
To find the value of kf (freezing point depression constant) for cyclohexane, we will use the formula:
∆T = kf * m
where ∆T is the change in freezing point, kf is the freezing point depression constant, and m is the molality of the solute.
Given:
∆T = 6.5 °C - 3.3 °C = 3.2 °C
m = moles of solute / mass of solvent in kg
First, we need to calculate the moles of benzene (C6H6):
Molar mass of benzene (C6H6) = 6(12.01 g/mol) + 6(1.01 g/mol) = 78.11 g/mol
moles of benzene = mass / molar mass = 1.00 g / 78.11 g/mol
Next, calculate the mass of cyclohexane in kg:
mass of cyclohexane = 80 g = 80/1000 kg
Now let's calculate the molality (m) of benzene in cyclohexane:
m = moles of solute / mass of solvent in kg
m = (1.00 g / 78.11 g/mol) / 0.080 kg
Now that we have the molality (m) and the change in freezing point (∆T), we can solve for kf (the freezing point depression constant):
kf = ∆T / m
kf = 3.2 °C / [(1.00 g / 78.11 g/mol) / 0.080 kg]
Calculating kf:
kf = 25.05 °C * kg / mol
Therefore, the value of kf (freezing point depression constant) for cyclohexane is approximately 25.05 °C * kg/mol.
To find the value of kf (freezing point depression constant) for cyclohexane, we can use the formula:
ΔT = kf * b * m
Where:
ΔT is the change in freezing point
b is the molality of the solute
m is the cryoscopic constant
In this case, the change in freezing point (ΔT) is 6.5 °C - 3.3 °C = 3.2 °C.
To find the molality (b) of the solute, we need to calculate the moles of benzene (C6H6) using its molar mass and the given mass of benzene (1.00 g).
The molar mass of benzene (C6H6) is:
C: 12.01 g/mol x 6 = 72.06 g/mol
H: 1.008 g/mol x 6 = 6.048 g/mol
Total molar mass of benzene (C6H6) = 72.06 g/mol + 6.048 g/mol = 78.108 g/mol
Now, let's calculate the moles of benzene (C6H6):
moles = mass / molar mass
moles = 1.00 g / 78.108 g/mol
Next, we need to calculate the molality (b) of the solute benzene (C6H6) in the solution. Molality is defined as moles of solute per kilogram of solvent.
First, we need to convert the mass of cyclohexane (80 g) to kilograms:
mass of cyclohexane (in kg) = 80 g / 1000 = 0.080 kg
Now, we can calculate the molality (b) of the solute benzene (C6H6):
molality (b) = moles of benzene (C6H6) / mass of cyclohexane (in kg)
Finally, we can plug in the values into the formula to find kf:
kf = ΔT / (b * m)
Substituting the given values:
ΔT = 3.2 °C
b = molality of benzene (C6H6)
m = cryoscopic constant for cyclohexane (constant value)
With this information, we can now proceed to calculate the value of kf for cyclohexane.
delta T = Kf*molality
mols benzene = grams/molar mass = ?
molality = mols benzene/kg cyclohexane
You know m and delta T, solve for Kf.