# Determining the boiling point of a substance given the triple point and critical point

If I have the pressure and temperature of a substance in its triple point, as well as its pressure and temperature in the critical point, how can I determine the boiling point of the substance at 1 atm of pressure?

• You can try using various functions of state - it's quite broad topic and one needs much more data about the substance to to do it with good precision. May 22 '15 at 21:20
• en.wikipedia.org/wiki/Equation_of_state May 22 '15 at 21:29

$$\ln{\left(\dfrac{P_1}{P_2}\right)}=\dfrac{\Delta H_{\text{vap}}}{R}\left(\dfrac{1}{T_2}-\dfrac{1}{T_1}\right)$$
Step 1: Find the enthalpy of vaporizations $$\Delta H_{\text{vap}}$$ by plugging in the two sets of temperatures and pressures that you have.
Step 2: Use your $$\Delta H_{\text{vap}}$$ and the temperature and pressure of the triple point to determine the boiling temperature at 1 atm.
• In principle, using the Clausius-Clapeyron equation is correct. However, the implied approximations of this approach ($\Delta_\text{vap} H = \text{const}$ and $\Delta_\text{vap} V\ = V(\mathrm{g})$) are not valid near the critical point. In the given question, one of the reference ($p,T$) points is the critical point; thus, the results might show a significant error.