Question

4. a compound called pyrene has the empirical formula c8h5. when 4.04 g of pyrene is dissolved in 10.00 g of benzene, the boiling point of the solution is 85.1°c calculate the molar mass of pyrene and determine its molecular formula. the molal boiling-point constant for benzene is 2.53°c/m. its normal boiling point is 80.1°c

Answer 1

The molecular mass of pyrene is 204.4 g/mol.

From;

ΔT = Kb m i

Where;

• ΔT = boiling point elevation
• Kb = boiling point constant
• m = molality
• i = Van't Hoff factor

Since the compound is molecular; i = 1

The number of moles of pyrene = 4.04 g/MM

Where; MM = molar mass of pyrene

molality = number of moles of pyrene/mass of solvent in Kg

The mass of solvent = 10 g or 0.01 Kg

molality =  4.04 g/MM/0.01

ΔT = Boiling point of solution - Boiling point of pure solvent

ΔT = 85.1°C - 80.1°C

ΔT = 5°C

5 = 2.53 × 4.04 g/MM/0.01 × 1

5 = 10.22 × 1/0.01 MM

0.05MM = 10.22

MM= 10.22/0.05

MM= 204.4 g/mol

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