Precision Calibrated Dumas Flasks
Heavy-walled borosilicate Erlenmeyer and round-bottom flasks with precision-calibrated internal volumes (tolerance ±0.1 mL) determined by brim-full water pycnometry.
Chemistry / Volatile Liquid Molar Mass
FREE TO EXPLORELABORATORY 05 · NATURAL SCIENCES
Molar Mass of a Volatile Liquid: The Dumas Vapor Density Method
Determine the molecular weight and molar mass of an unknown volatile organic liquid (cyclohexane) using the classical Dumas vapor density method, applying the ideal gas equation of state to vapor trapped at boiling water temperature and atmospheric pressure, correcting for buoyancy and vapor non-ideality, and extracting the molar mass via ordinary least-squares regression. Explore interactive analytical gravimetry, Class A volumetric glassware and density determination.
MEASURE · CALIBRATE · ANALYSE
Your virtual chemistry laboratory.
NET MASS OF CONDENSED VAPOR M_VAPOR—
MEASUREMENT STATUS
Ready to dispense 275.0 mL of liquid.
Adjust bulb volume V, then weigh the dispensed aliquot.
Mass observations are determined by the high-precision thermodynamic equation of state for pure liquid water. Readings reflect thermal equilibrium and analytical balance force restoration.
Physical chemistry assumptions and metrological limits
The determination of volatile liquid molar mass via the Dumas method is subject to specific physical constraints: (1) Real Gas Non-Ideality: Organic vapors near their condensation temperatures exhibit significant attractive intermolecular forces (van der Waals constant a for cyclohexane is 23.3 L²·bar/mol²), leading to compressibility factors Z ≈ 0.97–0.98; treating the vapor as ideal slightly overestimates molar mass by 1.5–2.5% unless corrected by the Berthelot or van der Waals equation. (2) Atmospheric Air Back-Diffusion: As the flask cools upon removal from the bath, internal vapor contracts and condenses, drawing room air in through the pinhole; this introduces ambient moisture unless weighed promptly. (3) Thermal Expansion of Borosilicate Glassware: The internal volume of the glass flask expands by ΔV/V = 3α·ΔT ≈ 3 · (3.3 × 10⁻⁶ K⁻¹) · (75 K) ≈ 0.07% between room temperature and 100 °C, which is minor but measurable. (4) Incomplete Flushing of Air: If an insufficient quantity of volatile liquid is added initially (<3 mL), the expanding vapor plume will not completely flush initial air from large bulbs. The Dumas physical model incorporates the ideal gas relationship m_vapor = [P_atm · M / (R · T_bath)] · V_flask. Operating conditions: T_bath = 373.15 K (100.0 °C), P_atm = 1.000 atm (101.325 kPa), and universal gas constant R = 82.0574 mL·atm/(mol·K). Under these conditions, the theoretical vapor density of cyclohexane (M = 84.161 g/mol) is ρ_ideal = 0.0027486 g/mL. Because cyclohexane vapor at 100 °C exhibits slight van der Waals intermolecular attraction (parameter a = 23.3 L²·bar/mol²), the real compressibility factor is Z ≈ 0.985, causing the uncorrected ideal gas formula to slightly overestimate molar mass by ~1.5%. Multi-point linear regression across varying bulb volumes completely absorbs constant aluminum foil tare offsets into the intercept m₀, while Archimedean air buoyancy compensation (accounting for the mass of air displaced by the empty flask tare) refines absolute gravimetric accuracy.
Analytical chemistry apparatus & tools.
Constant-Temperature Boiling Water Bath
2000 mL tall-form Pyrex beaker positioned on a high-capacity 1000 W digital ceramic hotplate, containing boiling deionized water with boiling stones to maintain a uniform 100.0 ± 0.1 °C temperature.
Pinhole Aluminum Foil Septum Assembly
Heavy-gauge aluminum foil cap crimped hermetically around the flask neck with copper wire, pierced at the exact center with a 0.5 mm tungsten needle to create a micro-orifice that vents air while preventing back-diffusion.
Submersion Flask Clamp & Retort Stand
Adjustable 3-prong vinyl-coated laboratory utility clamp and cast-iron support stand to rigidly immerse the Dumas flask submerged up to its neck in the boiling bath.
Fortin Mercury Barometer & NIST Digital Thermometer
Precision Fortin mercury barometer reading to ±0.1 mmHg, paired with a NIST-traceable immersion thermometer (resolution ±0.05 °C) to monitor bath temperature.
Four-Decimal Analytical Balance
High-precision electronic balance with 0.1 mg (0.0001 g) readability, anti-draft shield, and internal calibration to record empty tares and condensed liquid gross masses.
FOLLOW THE METHOD
A reason behind every step.
Determine the molecular weight and molar mass of an unknown volatile organic liquid (cyclohexane) using the classical Dumas vapor density method, applying the ideal gas equation of state to vapor trapped at boiling water temperature and atmospheric pressure, correcting for buoyancy and vapor non-ideality, and extracting the molar mass via ordinary least-squares regression. Suggested independent volumetric settings: 125.0, 175.0, 225.0, 275.0, 325.0, 375.0 mL.
01
Flask Conditioning, Septum Crimping, and Tare Weighing
Clean and thoroughly dry the borosilicate Dumas bulb inside a convection drying oven; crimp a square of heavy aluminum foil tightly over the flask rim with copper wire, puncture a single 0.5 mm pinhole in the center, and record the tare mass (m_tare) on the analytical balance to ±0.0001 g.
02
Volatile Sample Introduction
Remove the foil cap, introduce approximately 5.0 mL of pure volatile liquid (cyclohexane, bp 80.7 °C) into the flask using a transfer pipette, and re-crimp the punctured foil cap tightly with the wire ligature.
03
Submersion and Active Thermal Vaporization
Clamp the Dumas flask by its neck onto the retort stand and submerge it into the boiling water bath (100.0 °C) so that the bulb is completely immersed below the water line with only the foil cap protruding; the volatile liquid boils vigorously, generating a dense vapor stream that purges all residual air out through the pinhole.
04
Equilibration and Pinhole Sealing
Observe the flask through the boiling water until the last droplet of liquid vaporizes completely; allow the flask to remain in the bath for an additional 2 minutes to ensure the vapor achieves thermal equilibrium at T_bath = 100.0 °C and barometric pressure P_atm; record the bath temperature to ±0.05 °C and the barometric pressure to ±0.1 mmHg.
05
Quenching, Exterior Desiccation, and Condensation Weighing
Carefully remove the hot flask from the bath using insulated crucible tongs and immerse the lower half briefly in cool water to induce rapid condensation of the internal vapor; thoroughly dry the exterior glass surface with lint-free towels, ensure no droplet moisture remains under the foil crimp, and weigh the cooled flask (m_gross) on the analytical balance to ±0.0001 g; subtract m_tare to isolate condensed vapor mass m_vapor.
06
Pycnometric Volume Calibration and Regression Analysis
Remove the foil cap, fill the flask to overflowing with deionized water equilibrated to known temperature, slide a flat glass plate across the rim to eliminate air bubbles, dry the exterior, and weigh to determine the true internal volume V_flask from water density; repeat across six flasks (125 to 375 mL), tabulate data, and apply ordinary least-squares regression of m_vapor versus V_flask to extract the molar mass M.
YOUR NOTEBOOK · 0 OBSERVATIONS
Turn measurements into quantitative chemical insight.
Points are your recorded gravimetric observations. The regression line validates the linear mass-volume proportionality and extracts density.
ORDINARY LEAST-SQUARES FIT · m = ρ·V + m₀
m_vapor = (P_atm · M / [R · T_bath]) · V_flask + m₀ ==> m = 0.002749 · V
| # | Calibrated Dumas bulb volume V_flask (mL) (mL) | Net mass of condensed vapor m_vapor (g) | Dumas bulb volume V_flask (mL) | Net mass of condensed vapor m_vapor (g) | Remove |
|---|
No readings yet. Dispense and weigh a volumetric aliquot to record it.
Calculate it yourself.
CHECK YOUR UNDERSTANDING