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Heat of fusion (ΔHfus) is used for calculations involving a phase change between solid and liquid,...

Heat of fusion (ΔHfus) is used for calculations involving a phase change between solid and liquid, with no temperature change. For H2O, ΔHfus=6.02 kJ/mol. Specific heat capacity (C) is used for calculations that involve a temperature change, but no phase change. For liquid water, C=4.184 J/(g⋅∘C). Heat of vaporization (ΔHvap) is used for calculations involving a phase change between liquid and gas, with no temperature change. For H2O, ΔHvap=40.7 kJ/mol. How much heat is required to boil 77.5 g of water at its boiling point? Express your answer numerically in kilojoules.

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Answer #1

Number of moles of water = mass/mol.wt. = 77.5 g/(18.015 g/mol) = 4.301971 mol

Heat energy required to convert liquid water to water vapour is

q = m * ΔHvap = 4.301971 mol * 40.7 kJ/mol = 175.0902 kJ = 175.1 kJ

Heat energy required = 175 kJ

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