Question

How do you calculate the boiling point at different pressures?

Answer 1

Boiling point: The boiling point is termed as the temperature when the vapour pressure of a liquid becomes equal to the air pressure of the atmosphere above the liquid.

Boiling point at different pressure:

• If a specific substance undergoes variations in surrounding atmospheric pressures, the Boiling Points will decrease with decreasing atmospheric pressure values and vice-versa.
• If the boiling temperature and pressure are given for a certain substance, determining the boiling point at different values ​​of vapor pressure can be possible by using the Clausius Clapeyron equation.

Clausius Clapeyron equation: $\frac{\mathrm{d}(\ln \mathrm{P})}{\mathrm{dT}}=\frac{∆{\mathrm{H}}_{\mathrm{v}}}{{\mathrm{RT}}^2}$

$$\begin{gathered} \mathrm{P}=\mathrm{Pressure} \\ \mathrm{T}=\mathrm{Boiling}\mathrm{point} \\ ∆{\mathrm{H}}_{\mathrm{v}}=\mathrm{Latent}\mathrm{heat}\mathrm{of}\mathrm{vapourization}\mathrm{of}\mathrm{liquid} \\ \mathrm{R}=\mathrm{Gas}\mathrm{constant} \end{gathered}$$

The above equation can be written as: $\ln \left(\frac{{\mathrm{P}}_2}{{\mathrm{P}}_1}\right)=\frac{{\mathrm{\Delta H}}_{\mathrm{v}}}{\mathrm{R}}\left(\frac{1}{{\mathrm{T}}_1}-\frac{1}{{\mathrm{T}}_2}\right)$

$\log \left(\frac{{\mathrm{P}}_2}{{\mathrm{P}}_1}\right)=\frac{{\mathrm{\Delta H}}_{\mathrm{v}}}{2.303\mathrm{R}}\left(\frac{1}{{\mathrm{T}}_1}-\frac{1}{{\mathrm{T}}_2}\right)$

Hence, by applying the Claus Clapeyron equation above as a formula, we can calculate the boiling point at different pressures.