mathematics

What is the vapor pressure at 23°C of a solution of 1.20 g of naphthalene, C10H8, in 25.6 g of benzene,
C6H6? The vapor pressure of pure benzene at 23°C is 86.0 mmHg; the vapor pressure of naphthalene
can be neglected. Calculate the vapor-pressure lowering of the solution.

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  1. The Raoult's law says that the vapor pressure of a solvent in a solution is always lower than the vapor pressure of the pure solvent. The vapor pressure lowering is directly proportional to the mole fraction of the solute. In mathematical terms that means:

    ΔP = (1)

    ΔP is the vapor-pressure lowering of the solution is the molar fraction of the solute and is the vapor pressure of the pure solvent (86,0 mmHg)

    The moles of naphthalene are:
    9,36x10⁻³ moles solute

    The moles of benzene are:
    0,328 moles solvent

    Molar fraction of solute is:
    0,0278

    Replacing in (1)
    ΔP = 2,39 mm Hg

    Therefore, the vapor-pressure lowering of the solution is 2,39 mmHg

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