How much energy must be removed from a 125 g sample of benzene (C _6 H _6) vapor

Question

How much energy must be removed from a 125 g sample of benzene (C _6 H _6) vapor at 425.0 K to make it a solid at 200.0 K? Use the following steps to get to your answer. You MUST show your work to receive any credit. The following physical data for benzene may be useful. See announcement for due date. Delta Hvap = 33.9 kJ/mol delta Hfus = 9.8 kJ/mol C liq = 1.73 J/g degree C C gas = 1.06 J/g degree C C sol = 1.51 J/g degree C Timelting = 279.0K T boiling = 353.0K Determine number of moles the sample contains. __________moles How much energy must be removed to cool the gas to its boiling point? How much energy must be removed to convert the gas to a liquid ________ J? How much energy must be removed to bring the liquid from its _______J boiling point to its melting point?

Explanation / Answer

Answer(1)

molar mass of benzene = 78.11 g mol-1

The sample of benzene contains 125g / 78.11 g/mol = 1.60 moles.

Answer(2)

In cooling the gaseous sample from 425K to 353K (its boiling point)

Q = m·c·T = 125 x 1.06 x (425 - 353) = 9540J = 9.540 kJ

Answer(3)

Liquefying the sample at 353 K

Q = Hvap x 1.6 mol = 33.9 x 1.6 = 54.24 kJ

Answer(4)

The specific heat of the solid until you reach your desired temperature

Q = m·c·T = 125 x 1.51 x (353 - 279) = 13967J = 13.967 kJ

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