chemistry help?

2020-07-13 11:57 pm

When 11.4 g of an organic compound known to be 57.15% C, 4.8% H, and 38.06% O by mass is dissolved in 859.2 g of water, the freezing point is −0.196 ∘C. The normal freezing point of water is 0 ∘C. What is the molecular formula for the organic compound? Assume that the organic compound is a molecular solid and does not ionize in water.

回答 (1)

micatkie

2020-07-14 12:52 am

In the compound, mole ratio C : H : O
= (57.15/12) : (4.8/1) : (38.06/16)
= 4.8 : 4.8 : 2.4
= (4.8/2.4) : (4.8/2.4) : (2.4/2.4)
= 2 : 2 : 1

The empirical formula of the compound = C₂H₂O

Consider the freezing point depression of the solution:
Freezing point depression, ΔTf = [0 - (-0.196)] °C = 0.196 °C
van't Hoff factor, i = 1
Freezing point depression constant of water, Kf= 1.86 °C/m

ΔTf = i Kf m
0.196 = 1 × 1.86 × m
Molality, m = 0.196/1.86 = 0.105 mol/kg

Molar mass of the organic compound
= 11.4 g / [(0.105 mol/kg) × (859.2/1000 kg)]
= 126 g/mol

Let the molecular formula of the compound = (C₂H₂O)ₙ
Its molar mass in g/mol:
(12×2 + 1×2 + 16)n = 126
42n = 126
n = 3

The molecular formula of the compound = C₆H₆O₃

👍 3 👎 0