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Calculating Vapor Pressure of Ethanol

by BVV |

The following blog will show you how to effectively calculate the pressure a pure liquid will evaporate at with temperature being your known variable. Commonly known as the vapor pressure curve, all pure liquids have a set boiling temperature in relation to pressure. An increase in pressure will result in a higher boiling point, whereas decreasing the pressure or evacuating air from the atmosphere will lower the boiling point.

During rotary evaporation, we are trying to lower the boiling point of the solvent to make it easier and more efficient to recover. It’s important to effectively evaporate and re-condense your solvent; if not you will contaminate the pump and have a lower percentage solvent recovery. The equation we will be using is known as the Clausius-Clapeyron equation. It’s important to know that this equation can only be used for pure liquids. Pure liquids are those which cannot be further separated. This equation can’t be used with liquid mixtures, such as, gasoline because it’s composed of many hydrocarbons that can be separated through evaporation.

First, we must get familiar with ethanol also known as ethyl alcohol, 𝐶𝐻3−𝐶𝐻2−𝑂𝐻. We know that ethanol has an enthalpy of vaporization of 38,560 𝐽𝑚𝑜𝑙⁄ or 38.56 𝐾𝐽𝑚𝑜𝑙⁄ and a normal boiling point of 78.29°C. Next, we will determine a known temperature which the evaporating flask will experience during rotary evaporation. For this example, we will be using 50°C, but it can be changed to any desired temperature if the equipment is capable.

Clausius-Clapeyron Equation:


We now know that if we can effectively control the evaporating flask internal temperature at 50°C we will need to reach a vacuum of 238.4 torr to begin evaporating ethanol. This calculation can be done at any desired temperature; feel free to use this as a tool to compute the optimal temperature and pressure to operate your rotary evaporator. Precise vacuum regulators are sold on our website, please view links provided below.