3 – Measuring the Relative Molecular Mass of a Volatile Liquid (LC 2027) (Leaving Cert Chemistry): Revision Notes
3 – Measuring the Relative Molecular Mass of a Volatile Liquid
Introduction
This experiment provides a practical method for determining the relative molecular mass of volatile liquids using simple laboratory equipment. Unlike expensive mass spectrometers, this method uses basic principles of vapour behaviour and can be performed in most school laboratories. The technique is particularly useful for compounds that easily evaporate at relatively low temperatures.
This experimental approach offers an accessible alternative to sophisticated analytical instruments while maintaining good accuracy. The method is based on fundamental gas laws and can be applied to a wide range of volatile organic compounds commonly found in school laboratories.
Theory and key principles
The experiment is based on the relationship between the mass of a vapour, its volume, and its molecular mass. When a volatile liquid is heated, it converts to vapour, and by measuring the mass of this vapour and the volume it occupies, we can calculate the relative molecular mass.
Key concepts you need to understand:
- Volatile liquids have low boiling points and readily evaporate at room temperature
- Vapour pressure increases with temperature until it equals atmospheric pressure
- At the boiling point, the liquid rapidly converts to vapour
- The volume of vapour is much larger than the original liquid volume
- Atmospheric pressure affects the volume calculations
The fundamental principle underlying this experiment is that at a given temperature and pressure, equal volumes of gases contain equal numbers of molecules (Avogadro's Law). This allows us to relate the mass of vapour to its molecular mass through volume measurements.
Apparatus and materials
The experimental setup requires carefully selected equipment to ensure accurate measurements:
- A clean, dry conical flask
- Aluminium foil and rubber band for sealing
- Laboratory balance (capable of measuring to 0.01g)
- Thermometer for temperature measurement
- Volatile liquid sample (such as propanone or cyclohexane)
- Boiling water bath for controlled heating
- Measuring cylinder for volume determination
- Safety equipment including fume cupboard access
The choice of conical flask is important as it provides a known volume that can be easily measured. The aluminium foil must create an effective seal while allowing the crucial pinhole for pressure equilibration.
Detailed procedure
The experimental procedure must be followed precisely to ensure accurate results:
Step 1: Initial preparation
- Weigh the clean, dry conical flask along with aluminium foil and rubber band
- Record this initial mass accurately to the nearest 0.01g
Step 2: Adding the sample
- Pour approximately 10 cm³ of the volatile liquid into the flask
- The exact volume doesn't need to be precise at this stage
Ensure you add sufficient liquid to guarantee complete filling of the flask with vapour when heated. Too little liquid may result in incomplete vapourisation and inaccurate results.
Step 3: Sealing the flask
- Cover the flask mouth with aluminium foil
- Secure with a rubber band
- Make a small pinhole in the foil using a mounted needle
Step 4: Heating process
- Clamp the flask in a beaker of boiling water
- Ensure all the volatile liquid evaporates completely
- The vapour will fill the flask and excess vapour escapes through the pinhole
The heating process should be continued until no more liquid is visible in the flask and vapour can be seen escaping through the pinhole. This ensures that the flask is completely filled with vapour at the measured temperature.
Step 5: Temperature measurement
- Measure and record the temperature of the boiling water using a thermometer
- This represents the temperature of the vapour in the flask
Step 6: Cooling and final weighing
- Remove the flask from the boiling water
- Allow it to cool to room temperature
- Remove the foil and rubber band, then dry the outside
- Weigh the flask again - it now contains the mass of liquid that condensed from the vapour
Observations and results
During the experiment, you should observe several key phenomena that indicate the method is working correctly:
- The volatile liquid evaporating rapidly when heated
- Vapour escaping through the pinhole in the foil
- Droplets of liquid forming inside the flask as it cools
- The flask appears empty after cooling, but actually contains the condensed liquid
The formation of condensed droplets during cooling is a positive indicator that vapour was successfully trapped in the flask. The amount of liquid formed should be significantly less than the original volume, as only the mass equivalent to the vapour volume remains.
Key measurements needed:
- Initial mass of flask, foil, and rubber band
- Final mass after cooling
- Temperature of the boiling water
- Atmospheric pressure (usually 101.3 kPa)
Explanation of the method
The success of this experiment relies on several important principles that work together to provide accurate molecular mass determination:
Vapour behaviour: At the boiling water temperature, the volatile liquid completely vapourises and fills the flask. Any excess vapour escapes through the pinhole, ensuring the pressure inside equals atmospheric pressure.
Volume relationship: The volume of vapour in the flask equals the volume of the flask itself, which can be measured by filling it completely with water.
Mass calculation: The difference between the final and initial masses gives the mass of liquid whose vapour filled the flask at the measured temperature.
Pressure considerations: Since the vapour can escape through the pinhole, the pressure inside the flask equals atmospheric pressure, simplifying calculations.
The pinhole is absolutely critical to the success of this method. Without it, the pressure inside the flask would exceed atmospheric pressure, leading to incorrect volume-to-mass relationships and inaccurate molecular mass calculations.
Calculation process
To find the relative molecular mass, you need to apply gas law principles systematically:
- Calculate the mass of the volatile liquid from the mass difference
- Determine the volume of the flask by filling it with water
- Use the ideal gas equation or gas density relationships
- Account for the temperature and pressure conditions
The calculation involves using gas laws to relate the mass, volume, temperature, and pressure to determine the molar mass of the volatile substance.
Worked Example: Calculation Steps
Step 1: Calculate mass of vapour Mass of vapour = Final mass - Initial mass = 157.89g - 157.34g = 0.55g
Step 2: Determine flask volume Flask volume = 250 cm³ = 0.25 dm³
Step 3: Convert temperature to Kelvin Temperature = 100°C + 273 = 373 K
Step 4: Apply gas law relationship Using the relationship: Molar mass = (mass × R × temperature) / (pressure × volume)
Safety considerations
Working with volatile liquids requires careful attention to safety protocols:
Critical Safety Requirements:
- Work in a well-ventilated area as volatile liquids produce potentially harmful vapours
- Keep volatile liquids away from flames and heat sources when not needed
- Handle hot apparatus with appropriate equipment such as tongs or heat-proof gloves
- Wear safety glasses throughout the experiment
- Dispose of volatile liquids according to laboratory guidelines
Volatile organic compounds can pose health risks through inhalation or skin contact. Always check the safety data sheet for your specific compound and ensure appropriate ventilation is available throughout the procedure.
Advantages and limitations
Understanding both the strengths and weaknesses of this method helps in interpreting results accurately:
Advantages:
- Uses readily available laboratory equipment
- Cost-effective alternative to mass spectrometry
- Suitable for most volatile organic compounds
- Provides good accuracy when performed carefully
Limitations:
- Only works for volatile substances with appropriate boiling points
- Requires careful temperature and pressure measurements
- Small errors in mass measurement significantly affect results
- Not suitable for compounds that decompose when heated
The accuracy of this method depends heavily on precise measurements and careful technique. While it cannot match the precision of modern instrumental methods, it provides valuable understanding of molecular mass determination principles.
Applications and extensions
This experimental approach can be extended to explore various aspects of molecular behaviour:
- Comparing molecular masses of different volatile compounds
- Investigating the effect of temperature on vapour density
- Exploring deviations from ideal gas behaviour
- Understanding the relationship between molecular structure and volatility
Key Points to Remember:
- The pinhole in the foil is crucial - it allows excess vapour to escape and maintains atmospheric pressure inside the flask
- Temperature measurement must be accurate as it directly affects the calculations using gas laws
- The flask must be completely dry initially to ensure accurate mass measurements
- Complete vapourisation is essential - all liquid must convert to vapour for accurate results
- This method provides an accessible alternative to expensive mass spectrometry for determining molecular masses of volatile compounds
- Careful attention to safety protocols is essential when working with volatile organic compounds