Three pure substances—water, ethanol, and benzene—are each provided as 50.0 g samples at their respective melting points.
Data for the three substances (at 1 atm):
• Water: molar mass = 18.0 g mol⁻¹; molar enthalpy of fusion = 6.01 kJ mol⁻¹
• Ethanol: molar mass = 46.1 g mol⁻¹; molar enthalpy of fusion = 5.02 kJ mol⁻¹
• Benzene: molar mass = 78.1 g mol⁻¹; molar enthalpy of fusion = 9.95 kJ mol⁻¹
(a) Calculate the quantity of heat, in kJ, required to melt each 50.0 g sample completely at its melting point.
(b) Rank the three substances from greatest to least total heat absorbed in part (a). Briefly justify your ranking in terms of the data provided.
(c) Suppose instead that 1.00 mol of each substance is melted at its melting point. Which substance now requires the greatest amount of heat? Provide a quantitative comparison and explain why the ranking differs from that in part (b).