Answer:
a) 2.15x10⁻³ moles solute
b) The unknown compound could be p-dibromobenzene
Explanation:
The addition of an ideal solute to a solvent decreases its freezing point following the formula:
a) ΔT = Kf×m×i
Where ΔT is change in freezing point (2.7°C), Kf is freezing point depression constant for cyclohexane (20.0 °C kg/mol), m is molality of the solution (moles/kg) and i is Van't Hoff factor (1 for a non-dissociating solute)
Replacing:
2.7°C = 20.0°C×kg/mol mol solute / 0.01595kg×1
2.15x10⁻³ moles solute
b) Molar mass of the solute is:
0.5079g / 2.15x10⁻³moles = 236g/mol
Molar mass of p-difluorobenzene is 114g/mol, p-dicholorobenzene is 147g/mol, p-dibromobenzene is 236g/mol and p-diiodobenzene 330g/mol.
Thus, the unknown compound could be p-dibromobenzene
Answer:
a. 2.07×10⁻³ moles of solute
b. 244.9 g/mol
c. p-dibromobenzene, it is the nearest molar mass
Explanation:
Let's apply the colligative property of freezing point depression:
ΔT = Kf . m
where ΔT means Freezing T° of pure solvent - Freezing T° of solution
Kf is the cryoscopic constant and m, molality (moles of solute in 1kg of solvent)
Kf for the cyclohexane is 20.8 °C/m so, let's replace in the formula
2.7°C = 20.8°C/ m . m
2.7°C / 20.8 m/°C = 0.130 m
These are the moles in 1kg of solvent, but the mass of the solvent, is 15.95 g.
m = mol/kg → 0.130 m/kg . 0.01595kg = 2.07×10⁻³ moles
As this moles corresponds to 0.5079g, molar mass will be:
g/m → 0.5079 g / 2.07��10⁻³ moles = 244.9 g/mol
