Question #4b054

1 Answer
Ernest Z.
Sep 20, 2014

The molecular formula is C₁₂H₂₆O.

This is a boiling point elevation problem in which you use the boiling point elevation to determine the molar mass of the solute.

Step 1. Determine the molar mass.

We use the boiling point elevation expression

ΔTb=Kbm

whereΔTb is the change in boiling point, Kb is the molal boiling point elevation constant for the solvent, and m is the molality of the solute.

For benzene, Kb = 2.53 °C·kg·mol⁻¹ and Tb° = 80.10 °C.

ΔTb=Tb–T∘b = 80.78 °C – 80.10 °C = 0.68 °C

m=ΔTbKb=0.68 °C2.53 °C⋅kg⋅mol−1 = 0.27 mol⋅kg−1

So we have 0.100 kg × 0.27 mol1 kg = 0.027 mol of lauric acid

∴ Molar mass = 5 g0.027 mol = 190 g /mol

If you want to calculate the molar mass in one step. The formula is

Molar mass = KbΔTb × grams of solutekilograms of solvent

Molar mass = 2.53 °C⋅kg⋅mol−10.68°C × 5 g0.100 kg = 190 g/mol

Step 2. Calculate the molecular formula.

EF = C₁₂H₂₆O; so EF mass = 186.33 u

Molecular Mass = 190 u

MF = (EF)n

So Molecular mass = EF mass × n

and n=Molecular massEF mass = 190 u186.33 u=1.0≈1.

The MF is (C12H26O)n = (C12H26O)1 = C12H26O

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