Question

If 0.640 g of beautiful blue crystals of azulene is dissolve in 99 g of benzen, the resulting solutions boils at 80.23 degrees celsius. find the molecular formula?

Answer 1

This problem handles boiling-point elevation, which means we will use the formula:

ΔT = Kb * m

Where ΔT is the difference of Temperature between boiling points of the solution and the pure solvent (Tsolution - Tsolvent). Kb is the ebullioscopic constant of the solvent (2.64 for benzene), and m is the molality of the solution.

Knowing that benzene's boiling point is 80.1°C, we solve for m:

Tsolution - Tsolvent = Kb * m

80.23 - 80.1 = 2.64 * m

m = 0.049 m

We use the definition of molality to calculate the moles of azulene:

0.049 m = Xmoles azulene / 0.099 kgBenzene

Xmoles azulene = 4.87 x10⁻³ moles azulene

We use the mass and the moles of azulene to calculate its molecular weight:

0.640 g / 4.875 x10⁻³ mol = 130.28 g/mol

A molecular formula that would fulfill that molecular weight is C₁₀H₁₀. So that's the result of solving this problem.

The actual molecular formula of azulene is C₁₀H₈.