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When 13.1 g of an unknown, non-volatile, non-electrolyte, X was dissolved in 100. g of methanol,...

When 13.1 g of an unknown, non-volatile, non-electrolyte, X was dissolved in 100. g of methanol, the vapor pressure of the solvent decreased from 122.7 torr to 116 torr at 298 K. Calculate the molar mass of the solute, X

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Answer #1

Vapor Pressure of Pure Solvent = 122.7 torr

Vapor Pressure of Impure Solvent = 116 torr

Vapor Pressure of Impure Solvent = Mole fraction of Solvent * Vapor Pressure of Pure Solvent

Mole fraction of Solvent = 116/122.7 = 0.94539

Molar mass of Methanol (CH3OH) = 12 + 4 * 1 + 16 = 32 gm/mol

Number of moles of Methanol (CH3OH) =100/ 32 = 3.125 moles

Let the number of moles of solute be x

3.125/(3.125+x) = 0.94539

x = 0.18049 moles

Molar mass of Solute X = Mass given/ number of moles = 13.1/0.18049 = 72.577 gm/mol

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