Of state. Contents and Concepts 1. Comparison of Gases, Liquids, and Solids Changes of State

Problem Strategy The two-point form of the Clausius–Clapeyron equation relates five quantities: P₁, T₁, P₂, T₂, and _H_vap. The normal boiling point is the temperature at which (continued)

11.2 Phase Transitions

t he vapor pressure equals 760 mmHg. Thus, you can let P₁ equal 760 mmHg and T₁ equal 373 K (100°C). Then P₂ would be the vapor pressure at T₂, or 358 K (85°C). The heat of vaporization, H_vap, is 40.7 * 10³ J/mol. Note that we have expressed this quantity in joules per mole to agree with the units of the molar gas constant, R, which equals 8.31 J/(K_mol).

Solution You can now substitute into the two-point equation (the equation shaded in color just before this example).


Y ou now want to solve this equation for P₂, which is the vapor pressure of water at 85°C. To do so, you take the antilogarithm (antiln) of both sides of the equation. This gives
The antiln (_0.550) is e^-0.550 = 0.577. (Appendix A describes how to obtain the

a ntilogarithm.) Thus

Therefore,


This value may be compared with the experimental value of 434 mmHg, given in Appendix B. Differences from the experimental value stem from the fact that the Clausius–Clapeyron equation is an approximate relation.