A chemistry student needs to determine the conditions at which a pure substance will change from solid to liquid phase. The substance in question has a melting point of 120 °C and a heat of fusion of 35 kJ/mol. How much heat is required to melt 100 grams of the substance at its melting point? And, what is the temperature at which the substance will start to solidify if the ambient pressure is 1 atm and the substance has cooled from its liquid phase at 150 °C to its solid phase?

Question

A chemistry student needs to determine the conditions at which a pure substance will change from solid to liquid phase. The substance in question has a melting point of 120 °C and a heat of fusion of 35 kJ/mol. How much heat is required to melt 100 grams of the substance at its melting point? And, what is the temperature at

Answer 1

First, we need to determine the amount of heat required to melt 100 grams of the substance at its melting point. To do this, we need to know the molar mass of the substance. Unfortunately, the molar mass is not provided in the problem. However, we can still express the heat required in terms of the molar mass.Let's denote the molar mass of the substance as M  g/mol . To find the number of moles  n  in 100 grams of the substance, we can use the formula:n = mass / molar massn = 100 g / MNow, we can use the heat of fusion  Hfus  to calculate the heat  q  required to melt the substance:q = n * Hfusq =  100 g / M  * 35 kJ/molSo, the heat required to melt 100 grams of the substance at its melting point is  3500/M  kJ, where M is the molar mass of the substance in g/mol.For the second part of the question, the temperature at which the substance will start to solidify is the same as its melting point under the given conditions  ambient pressure of 1 atm . This is because the melting point and freezing point are the same for a pure substance under constant pressure. Therefore, the substance will start to solidify at 120 C.