Stepwise Procedure and Formula for the Victor Meyer Experiment
The Determining Molecular Masses Using Victor Meyer’s Method is a classic laboratory technique to find the molecular mass of volatile organic compounds. Developed by Victor Meyer in the late 19th century, this experiment is widely included in the JEE Main Chemistry syllabus due to its conceptual clarity and practical exposure. The approach relies on Avogadro’s law and volumetric displacement of air, offering a reliable method for molecular mass determination in the lab.
Principle and Theory Behind Victor Meyer’s Method
At its core, the Victor Meyer method works on the principle that equal volumes of any gas, under the same temperature and pressure, contain the same number of molecules (Avogadro's Law). A known mass of a volatile substance is heated in a special tube, causing it to vaporise. This vapor displaces an equivalent volume of air, which is then measured to determine the number of moles in the original sample.
- The displaced air volume equals the vapor volume of the sample at the experiment’s conditions.
- This volume is corrected to standard temperature and pressure (STP: 273 K, 1 atm) for calculations.
- By comparing the volume at STP, the molar mass of the substance is deduced using proportion.
The formula typically used in the Victor Meyer experiment is: M = (w × 22400) / V, where w is the mass of the sample (g), V is the corrected vapor volume at STP (mL), and 22400 is the molar volume of a gas at STP (in mL).
Experimental Setup and Stepwise Procedure
The apparatus consists of the Victor Meyer tube, jacket for uniform heating, a side tube leading into a water trough, and a graduated measuring tube to collect the displaced air. Ensuring the experimental setup is airtight and the sample is dry are key precautions. Follow these steps for the experiment:
- Weigh a dry, small sample (w g) in a thin glass bulb or ampoule.
- Heat the Victor Meyer tube using a boiling liquid in the jacket—select one whose boiling point is at least 30 °C higher than the sample.
- Drop the ampoule containing the sample into the Victor Meyer tube rapidly and close the stopper.
- The sample vaporises instantly, displacing an equal amount of air, which passes through the side tube and is collected in the inverted graduated tube over water.
- Record the volume (Vobs) of air collected, the atmospheric pressure (P), temperature (T in K), and aqueous tension for corrections.
- Reduce the measured volume to STP (273 K, 1 atm) using the combined gas law for accurate molar mass determination.
Common precautions include ensuring the ampoule is dry, the system is leak-proof, no water enters the Victor Meyer tube, and the displaced air volume is measured at eye level to minimise parallax error.
Derivation and Application of the Victor Meyer Formula
To connect observed data to the actual molecular mass, correct the observed air volume to STP using:
- Combined Gas Law: VSTP = (Vobs × (P − Aqueous tension) × 273) / (760 × T)
Once VSTP is known, calculate the molecular mass (M):
- M = (w × 22400) / VSTP
Where 22400 mL is the molar volume at STP, w is the sample mass in grams, VSTP is the corrected volume in mL. This simple formula is essential for JEE Main calculations and matches vapour density approaches in related problems.
| Symbol | Quantity | Unit |
|---|---|---|
| w | Mass of substance taken | g |
| VSTP | Corrected vapor volume at STP | mL |
| M | Molecular mass | g/mol |
Worked Examples Using Victor Meyer’s Method
Below are example calculations for commonly tested substances using the Victor Meyer method. Assume atmospheric pressure of 760 mmHg, negligible aqueous tension, T = 273 K unless specified.
| Substance | Sample mass w (g) | Displaced air V (mL) | Calculated M (g/mol) | True M (g/mol) |
|---|---|---|---|---|
| H2 | 0.089 | 1000 | 2.0 | 2.0 |
| O2 | 1.43 | 1000 | 32.0 | 32.0 |
| Cl2 | 2.95 | 1000 | 71.0 | 71.0 |
| CH4 | 0.715 | 1000 | 16.0 | 16.0 |
| CO2 | 1.96 | 1000 | 44.0 | 44.0 |
| C2H6 | 1.34 | 1000 | 30.0 | 30.0 |
| NH3 | 0.765 | 1000 | 17.0 | 17.0 |
| CH3OH | 1.44 | 1000 | 32.0 | 32.0 |
Notice that the experimentally determined values closely match the true molecular masses, showcasing the accuracy of this approach in practical lab environments and JEE Main numericals.
Comparison with Other Molecular Mass Determination Methods
It is important for JEE aspirants to distinguish the Victor Meyer method from alternative techniques like the Dumas method. Here’s a concise comparison:
| Method | Key Feature | Limitations | Best Use Case |
|---|---|---|---|
| Victor Meyer | Displacement of air by vaporised sample | Only works for volatile substances; not for compounds that decompose on heating | Volatile liquids and low-boiling solids |
| Dumas | Direct measurement of vapor mass and volume | Needs careful evacuation; more complex setup | Compounds stable at higher temperatures |
- The Victor Meyer method is generally faster, safer, and suitable for educational labs compared to Dumas.
- It cannot be used for substances that undergo thermal decomposition before vaporising.
Tips, Applications and Common Pitfalls in Victor Meyer’s Method
For JEE Main practicals and theoretical questions, students should remember:
- Always use a sample that is fully volatile at the boiling liquid’s temperature.
- Correct all observed volumes for atmospheric pressure, temperature, and aqueous tension.
- Avoid parallax and air leaks in measuring apparatus to ensure reliable data.
- The method is still widely referenced in laboratories and in competitive exam problems, especially those linking Avogadro’s law, vapor density, and stoichiometry.
For deeper understanding of Avogadro’s law and related principles, see Boyle’s law and gas laws in JEE Chemistry. To practice related calculations and compare with similar topics, explore the vapour density method, mole concept, and Dumas’ method for molecular mass on Vedantu.
In summary, the Determining Molecular Masses Using Victor Meyer’s Method allows rapid and reliable molecular mass calculations for volatile compounds, integrates strongly with fundamental gas laws, and remains a staple in the JEE Main Chemistry toolkit.
FAQs on Determining Molecular Masses Using Victor Meyer’s Method
1. What is the Victor Meyer method?
The Victor Meyer method is an experimental technique used to determine the molecular mass of volatile substances by measuring the volume of vapor displaced on vaporization.
Key points:
- It relies on Avogadro’s law and relates the mass of a known sample to the volume of its vapor at standard conditions.
- Commonly included in JEE Main and CBSE exam syllabi for understanding experimental determination of molecular mass.
- The apparatus uses displacement of air, allowing calculation based on vapor volume.
2. What is the formula for the Victor Meyer method?
The main formula for the Victor Meyer method is:
- M = (w × 22.4 × 1000) / (V × d × t)
- M = Molecular mass of the substance
- w = Mass of the sample in grams
- V = Volume of vapor at laboratory conditions (in mL)
- d = Density of air
- t = Temperature in Kelvin (often standardised at STP for calculation)
3. What is the principle of the Victor Meyer test?
The principle of the Victor Meyer test is based on Avogadro's law, stating that equal volumes of gases at the same temperature and pressure contain the same number of molecules.
Main points:
- A known mass of a volatile substance, when vaporized, displaces an equal volume of air in the apparatus.
- The displaced air volume corresponds to the vapor produced by the substance.
- The volume of vapor equates to molecular mass, using gas laws for calculation.
4. How is the Victor Meyer method performed step by step?
To perform the Victor Meyer experiment, follow these steps:
- Place a small known mass of the volatile liquid (in a tiny tube) inside the Victor Meyer tube, previously heated in a vapor bath.
- Seal and quickly drop the tube into the apparatus.
- The liquid vaporizes instantly, displacing an equal volume of air which is collected over water in a graduated tube.
- Measure the volume of displaced air/vapor at laboratory conditions.
- Correct the measured volume to standard temperature and pressure (STP).
- Apply the Victor Meyer formula to calculate molecular mass.
5. Which substances can be analysed with the Victor Meyer method?
The Victor Meyer method is suitable for determining the molecular mass of volatile liquids and solids that vaporize without decomposition.
Common examples include:
- H₂, O₂, Cl₂, CO₂, CH₄, C₂H₆, C₂H₄, NH₃, CH₃OH
- Other organic liquids with appropriate volatility
6. What are the limitations of the Victor Meyer method?
The Victor Meyer method has several limitations:
- Applicable only to substances that vaporize without decomposition.
- Accuracy can be affected by errors in volume measurement or uncontrolled temperature/pressure.
- Not suitable for non-volatile or thermally unstable compounds.
- Requires correction for temperature and pressure deviations from standard conditions.
7. How is molecular mass calculated using the Victor Meyer apparatus?
Molecular mass is calculated by measuring the mass of substance and the volume of vapor it displaces in the Victor Meyer tube and applying the standard formula.
Procedure:
- Find mass of sample (w).
- Record volume of displaced vapor (V) at lab conditions.
- Convert volume to STP using gas law corrections.
- Apply the formula: M = (w × 22.4) / VSTP to obtain molecular mass.
8. What is the difference between the Victor Meyer and Dumas methods?
The Victor Meyer and Dumas methods both determine molecular mass of volatile substances, but differ in setup and principle:
- Victor Meyer: Measures displaced air volume, suitable for substances volatile below 200°C, safer and faster.
- Dumas Method: Involves vaporization in a sealed bulb and direct weighing, less suitable for thermally sensitive substances.
9. Can the Victor Meyer method be used for non-volatile substances?
No, the Victor Meyer method cannot be used for non-volatile substances.
This experiment requires the sample to vaporize readily without decomposition, so only volatile liquids and solids can be analyzed with this method.
10. What common errors should be avoided when performing the Victor Meyer method in practical exams?
Students should avoid these common errors during Victor Meyer experiments:
- Not drying the internal tube thoroughly before use.
- Measuring vapor volume at incorrect temperature or pressure.
- Carelessly sealing or breaking the sample tube, leading to leakage.
- Failing to correct the measured volume to STP.
- Using a sample that decomposes instead of fully vaporizing.
