Analysis of an Unknown Alcohol Lab Experiment
Safety note (tert-butyl alcohol / tert-butanol, CAS 75-65-0): PubChem’s hazard summary for tert-butanol (CID 6386, CAS 75-65-0) classifies it as a highly flammable liquid and vapour (H225), as causing serious eye irritation (H319), as harmful if inhaled (H332), and as possibly causing respiratory irritation (H335). This lab belongs in a fume hood, away from ignition sources, with eye protection and gloves; see also a manufacturer SDS such as Fisher Scientific tert-Butanol.
Analysis:
Utilizing the Lab Pro software, we were able to observe a very accurate real-time change in the freezing point of the T-Butyl alcohol after adding an unknown. This change in freezing point was used to calculate the molality of the unknown, as well as it’s molar mass by using it’s measured weight.
Comparing the molar mass of the unknown to the chart, it was obvious that the unknown was Isopropyl alcohol, which has a molar mass of 60.1 (g/mol), where our experimental results, 58.9 (g/mol), showed a mere 2% error.
The experiment was very accurate, however, there were observable sources of error. Mainly, a period of supercooling in the solution is thought to have accounted for the bulk of error. Other sources of error could be in calibration of equipment or impurities in materials
How freezing-point depression identifies the unknown
Freezing-point depression is a colligative property: for a dilute solution of a nonvolatile nonelectrolyte, the drop in freezing point depends on how many solute particles are present, not on which compound they are. OpenStax writes that relationship as
ΔTf = Kf · m
where ΔTf is the freezing-point depression (pure solvent freezing point minus solution freezing point), Kf is the solvent’s freezing-point depression constant, and m is the molality of the solute (moles of solute per kilogram of solvent). Source: OpenStax Chemistry 2e, 11.4 Colligative Properties.
In this experiment the solvent is tert-butyl alcohol. Lab Pro recorded the freezing point of the pure solvent and then of the solvent plus a weighed mass of unknown. From the measured ΔTf and the known Kf for the solvent, molality follows as m = ΔTf / Kf. With the measured mass of unknown and the mass of solvent used, molality converts to moles of unknown, and molar mass is
molar mass = (grams of unknown) / (moles of unknown).
Worked outline using this post’s numbers only. The lab report does not reprint the raw ΔTf, Kf, or mass values, but it does report the experimental molar mass and the chart comparison:
- Experimental molar mass from the freezing-point data: 58.9 g/mol.
- Chart value for isopropyl alcohol: 60.1 g/mol.
- Percent error: |58.9 − 60.1| / 60.1 × 100% ≈ 2%, matching the report.
Because 58.9 g/mol is closest to isopropyl alcohol on the chart (and well away from alcohols with much higher or lower molar masses), the unknown was identified as isopropyl alcohol. The report attributes most of the remaining error to a period of supercooling before the mixture froze, which can shift the apparent freezing point if the plateau is read from a supercooled region rather than from the true solid–liquid equilibrium temperature.