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    There are two gas parcels of equal volume, A and B at the same temperature and pressure. Parcel A is one mole of water vapour, while parcel B is one m
    Question

    There are two gas parcels of equal volume, A and B at the same temperature and pressure. Parcel A is one mole of water vapour, while parcel B is one mole of day air. Which of the following is TRUE?

    A.

    Parcel A is heavier then Parcel B

    B.

    Parcel B is heavier then Parcel A

    C.

    Both parcels are equally heavy

    D.

    Without temperature and pressure data, their relative masses cannot be determined

    Correct option is B

    Given:

    • Two gas parcels (A and B)

    • Same volume, temperature, and pressure

    • Parcel A:1 mole of water vapour (H₂O)

    • Parcel B:1 mole of dry air

    Concept Used:
    At the same temperature and pressure, the mass of one mole of a gas is simply its molar mass.

    • Molar mass of water vapour (H₂O) = 18 g/mol

    • Molar mass of dry air (approximate) = 78% N₂ (28 g/mol) + 21% O₂ (32 g/mol)
      → Weighted average ≈ 29 g/mol

    Solution:

    • Parcel A (water vapour): 18 g

    • Parcel B (dry air): 29 g

    So, Parcel B is heavier than Parcel A

    Correct Option: (B) Parcel B is heavier than Parcel A

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