PREVIOUSLY ASKED IN:
WBPSC Miscellaneous Preliminary 2019
Answer
64
Explanation
According to Avogadro's hypothesis, the molecular weight (M) of any gas is strictly equal to twice its vapour density (VD). The mathematical relationship is expressed as: Molecular Weight (M) = 2 × Vapour Density (VD). Given that the vapour density is 32, the molecular weight is simply 2 × 32 = 64. A common example of a gas with a molecular weight of 64 is Sulfur Dioxide (SO2).
Key Points
- > Vapour density is defined as the ratio of the mass of a certain volume of a gas to the mass of an equal volume of hydrogen gas under the same conditions of temperature and pressure.
- > Vapour density is a unitless ratio.
- > Hydrogen gas (H2) has a molecular weight of 2, so its vapour density is 1.
- > Oxygen gas (O2) has a molecular weight of 32, which gives it a vapour density of 16.
- > Avogadro's number is 6.022 × 10^23, representing the number of constituent particles (usually atoms or molecules) in one mole of a given substance.
Additional Information
Common Gases and their Vapour Densities
| Gas Name | Formula | Molecular Wt (M) | Vapour Density (M/2) |
|---|---|---|---|
| Hydrogen | H2 | 2 | 1 |
| Oxygen | O2 | 32 | 16 |
| Carbon Dioxide | CO2 | 44 | 22 |
| Sulfur Dioxide | SO2 | 64 | 32 |
| Ammonia | NH3 | 17 | 8.5 |
Memory Tips
- Formula Rule: Just remember M = 2 × VD.
- If the question asks for Molecular weight, multiply by 2. If it asks for Vapour Density, divide the molecular weight by 2.