Vapor Pressure Calculator

This calculator relies on two primary equations depending on your inputs. For temperature and pressure changes of a pure substance, we use the Clausius-Clapeyron equation: ln(P1 / P2) = (delta H_vap / R) * (1/T2 - 1/T1), where P represents pressure, T represents absolute temperature in Kelvin, delta H_vap is the molar enthalpy of vaporization, and R is the ideal gas constant (8.3145 J/(mol*K)). For solutions containing non-volatile solutes, we apply Raoult's Law: P_solution = P_solvent * X_solvent, where X represents the mole fraction of the solvent.

Imagine you want to find the new vapor pressure of ethanol when its temperature rises from 298 K (25 degrees Celsius) to 350 K. Assume ethanol has a molar enthalpy of vaporization of 38.56 kJ/mol (or 38560 J/mol) and an initial vapor pressure of 7.87 kPa at 298 K. Plugging these into the Clausius-Clapeyron equation: ln(7.87 / P2) = (38560 / 8.3145) * ((1 / 350) - (1 / 298)). Solving the right side yields approximately -2.285. Exponentiating both sides gives 7.87 / P2 = 0.1018, resulting in a final vapor pressure P2 of approximately 77.31 kPa.

Always convert Celsius or Fahrenheit temperatures to Kelvin by adding 273.15 before performing logarithmic calculations. Ensure your units match consistently—convert kilopascals to Pascals or Joules to kilojoules if your gas constant scale demands it. Remember that the Clausius-Clapeyron relation assumes enthalpy of vaporization remains constant across the chosen temperature interval.