The correct option for the definition of vapour density of a gas is **B. the mass of a unit volume of the gas compared to an equal volume of hydrogen**. ### Detailed Explanation: 1. **Understanding Vapour Density**: - Vapour density is a measure that compares the density of a gas to the density of hydrogen under the same conditions of temperature and pressure. It is defined as the mass of a certain volume of the gas divided by the mass of an equal volume of hydrogen. 2. **Why Option B is Correct**: - The vapour density (VD) is calculated using the formula: \[ \text{Vapour Density} = \frac{\text{Mass of gas}}{\text{Mass of equal volume of hydrogen}} \] - Hydrogen is chosen as the reference gas because it is the lightest and has a molecular weight of approximately 2 g/mol (H₂). This makes it a convenient baseline for comparison. - Therefore, when we say that the vapour density of a gas is compared to hydrogen, we are essentially saying that we are looking at how heavy the gas is relative to hydrogen, which helps in understanding its properties and behavior. 3. **Why the Other Options are Incorrect**: - **Option A: the mass of a unit volume of the gas compared to an equal volume of water vapour**: - This option is incorrect because water vapour is not the standard reference for vapour density. Water vapour has a molecular weight of about 18 g/mol, which is significantly heavier than hydrogen. Using water vapour would not provide a consistent basis for comparison across different gases. - **Option C: the mass of a unit volume of the gas compared to an equal volume of oxygen**: - This option is also incorrect. While oxygen (O₂) has a molecular weight of about 32 g/mol, it is not the standard reference for vapour density. Like water vapour, using oxygen would lead to inconsistencies in comparisons, as it is heavier than hydrogen. - **Option D: the mass of a unit volume of the gas minus the vapour pressure of water**: - This option is incorrect because it does not define vapour density. Vapour pressure is a separate concept that refers to the pressure exerted by a vapor in equilibrium with its liquid or solid form. Subtracting vapour pressure from mass does not yield a meaningful measure of density. ### Common Pitfalls: - Students often confuse vapour density with other related concepts such as molar mass or density. It is crucial to remember that vapour density specifically compares the mass of a gas to hydrogen. - Another common mistake is to use water vapour or oxygen as reference points, which can lead to incorrect conclusions about the properties of gases. ### Revision Summary: - Vapour density is defined as the mass of a gas compared to an equal volume of hydrogen. - The correct reference for vapour density is hydrogen due to its low molecular weight. - Other options (water vapour, oxygen, and vapour pressure) do not provide a valid basis for comparison in the context of vapour density. - Understanding the correct definition and reference gas is essential for solving problems related to vapour density in chemistry.