In the industrial production of sulphuric acid, SO_{3} is carefully reacted with water:
SO_{3} + H_{2}O → H_{2}SO_{4}
any excess SO_{3} giving rise to what is called oleum, a solution of SO_{3} in sulphuric acid. The relationship between the freezing point (f.p.) of sulphuric acid and the presence of small amounts of dissolved SO_{3} is as below:
Pure H_{2}SO_{4}
1 mol% SO_{3}
2 mol% SO_{3}
3 mol% SO_{3}
T_{fp} /°C
10.36
9.74
9.12
8.50
Assume that SO_{3} in H_{2}SO_{4} forms a solution without reaction. Graphically or otherwise, estimate the enthalpy of fusion of sulphuric acid (note that 1 mole % corresponds to a mole fraction of 0.01).