Calculate the freezing point of 0.5 g of biphenyl in 20.0 mL of cyclohexane. Cyclohexane Fp = 6.5 degrees C Kf 20.01
mols biphenyl = grams/molar mass = ?
molality biphenyl = mols/kg solvent
delta T = Kf*m. Solve for delta T.
Then subtract delta T from 6.5 to find the new freezing point.
To calculate the freezing point of the mixture, you need to use the formula for freezing point depression:
ΔT = Kf * m
Where:
ΔT is the change in freezing point
Kf is the freezing point depression constant for the solvent (cyclohexane)
m is the molality of the solute (biphenyl) in the solvent
First, we need to calculate the molality of biphenyl in the cyclohexane solution. Molality (m) is defined as the number of moles of solute per kilogram of solvent.
To find the molality, we need to calculate the number of moles of biphenyl and the mass of cyclohexane.
1. Calculate the number of moles of biphenyl:
Number of moles = Mass / molar mass
The molar mass of biphenyl (C12H10) is calculated as follows:
molar mass of C = 12.01 g/mol
molar mass of H = 1.01 g/mol
molar mass of biphenyl = (12.01 x 12) + (1.01 x 10) = 154.23 g/mol
Number of moles of biphenyl = 0.5 g / 154.23 g/mol
2. Calculate the mass of cyclohexane:
The density of cyclohexane is needed to convert the volume to mass. The density of cyclohexane is typically around 0.78 g/mL.
Mass of cyclohexane = Volume x Density = 20.0 mL x 0.78 g/mL
Now that we have the moles of biphenyl (solute) and the mass of cyclohexane (solvent), we can calculate the molality:
Molality (m) = moles of solute / kg of solvent
Molality (m) = moles of solute / (mass of cyclohexane / 1000)
Next, we can substitute the values into the freezing point depression formula:
ΔT = Kf * m
Now we can calculate the change in freezing point (ΔT).
Finally, to find the freezing point of the mixture, subtract the change in freezing point from the freezing point of the pure cyclohexane:
Freezing point of mixture = Freezing point of pure cyclohexane - ΔT