Experiment 18: Relative Molecular Mass of a Volatile Liquid
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Topic: Physical Chemistry / Gas Laws
Level: Ordinary & Higher Level
Exam Focus: Molar Mass Calculation
1. Experiment Snapshot
Feature
Details
Apparatus
Small conical flask or Victor Meyer-style setup, foil with pinhole, hot water bath, balance, thermometer, measuring cylinder, barometer or room pressure value, beaker, clamp stand.
Chemicals/Reagents
Volatile liquid sample, hot water, drying tissue.
Main observation
Liquid vaporises and escapes through the pinhole; after cooling, the mass loss is used in calculation.
Main measurement
Mass of vapour, volume of flask, temperature and pressure.
2. Aim
To determine the relative molecular mass of a volatile liquid by vaporising a known mass and using its vapour volume, temperature and pressure.
3. Quick Theory
A volatile liquid vaporises easily. If the mass, volume, temperature and pressure of its vapour are known, the number of moles can be calculated using the ideal gas equation, then relative molecular mass can be found.
PV = nRT and Mr = mass ÷ moles
Examiner Tip: Use Kelvin for temperature and correct SI units if using PV = nRT.
4. Apparatus, Chemicals and Safety
Category
Details
Apparatus
Small conical flask or Victor Meyer-style setup, foil with pinhole, hot water bath, balance, thermometer, measuring cylinder, barometer or room pressure value, beaker, clamp stand.
Chemicals
Volatile liquid sample, hot water, drying tissue.
Safety: Volatile liquids may be flammable and harmful. Keep away from naked flames where possible, use a water bath, wear goggles and work in a ventilated area.
5. Labelled Experiment Diagram
Relative Molecular Mass of a Volatile Liquid — Premium Practical Setup
6. Procedure
Weigh a clean dry flask covered with foil containing a tiny pinhole.
Add a small amount of volatile liquid and reweigh.
Place the flask in a hot water bath so the liquid vaporises.
Allow excess vapour to escape through the pinhole until vaporisation is complete.
Remove, cool and dry the outside of the flask.
Weigh the flask again to find the mass of vapour remaining.
Find the volume of the flask by filling it with water and measuring the water volume.
Record the water bath temperature and atmospheric pressure.
Use PV = nRT to calculate moles of vapour, then Mr = mass/moles.
Examiner Tip: Do not use room temperature for the vapour; use the temperature of the hot water bath.
7. Observations and Results
Stage / Test
Observation
Conclusion
Mass measurements
Mass before and after heating recorded
Mass of vapour found by difference
Volume of flask
Measured by filling with water
This equals vapour volume
Temperature
Measured in water bath
Convert to Kelvin
Pressure
Atmospheric pressure used
Needed for PV = nRT
Conclusion: The relative molecular mass is calculated from the mass of vapour and the number of moles found using the gas equation.
8. Calculations / Key Chemical Reasoning
This experiment is calculation-heavy. Convert cm³ to m³ if using SI units, and convert °C to K.
T(K)=°C+273 ; n=PV/RT ; Mr=mass/n
Worked Example: If n = 0.0020 mol and mass of vapour = 0.092 g, then Mr = 0.092 ÷ 0.0020 = 46.
9. Sources of Error and Improvements
Source of Error
Effect
Improvement
Vapour escapes during cooling
Mass measured too low
Cool carefully and weigh quickly
Flask not dry outside
Mass measured too high
Dry outside before weighing
Temperature incorrect
Wrong moles calculated
Use water bath temperature
Pinhole too large
Too much vapour escapes
Use a small pinhole
Units not converted
Calculation error
Use consistent SI units
10. Student Common Mistakes
Using Celsius instead of Kelvin.
Forgetting to measure flask volume.
Using liquid volume instead of vapour volume.
Not drying the flask before weighing.
Forgetting Mr = mass divided by moles.
11. Examiner Tips
Tip: Record all measurements clearly.
Tip: Use PV = nRT carefully with units.
Tip: Cool and dry before final weighing.
Tip: Mass of vapour is found from mass difference.
12. Examiner Traps
Trap: The vapour volume is the flask volume, not the liquid volume.
Trap: Temperature must be in Kelvin.
Trap: A volatile liquid is not necessarily a gas at room temperature.
13. 🎮 Interactive Simulator
Volatile Liquid Mr Calculator
Compare weak, correct and overheated conditions for vapour measurement.
Observation Window
Vapour setup
Choose a condition and run the simulation.
Lab Score: 0/0
14. Past Paper Style Questions
State the aim of this experiment. [2 marks]
Name the main apparatus or reagent used. [2 marks]
Describe the key observation expected. [2 marks]
Give one safety precaution. [1 mark]
Give one source of error and one improvement. [2 marks]
Explain the chemistry behind the result. [3 marks]
15. Mark Scheme Answers
Q1. To determine relative molecular mass of a volatile liquid using vapour data [2].
Q2. Flask with foil/pinhole, balance, thermometer, water bath, pressure/volume measurement [2].
Q3. Liquid vaporises and mass difference is measured [2].