Solvent Activity and Vapor Pressure: Pure-Solvent Data Need a Mixture Boundary

Define the Phase or Process Boundary

IUPAC's Raoult-law formulation relates equilibrium component fugacity to its activity and pure-component reference. Under a stated ideal-vapor approximation, solvent partial pressure can be written p=a p*. Activity is dimensionless and must retain its reference state; it is not automatically equal to concentration.

Review a Hypothetical Comparison

Assume an invented calculation at one temperature supplies pure-solvent vapor pressure p*=50 kPa and solvent activity a=0.70. The approximate solvent partial pressure is 35 kPa. Substituting 50 kPa would instead describe the pure-solvent reference. Substituting a mass fraction of 0.70 happens to give the same number only by coincidence unless a model actually supports that substitution. The other volatile components, if present, still contribute their own partial pressures.

Keep the Evidence with the Decision

Keep the mixture composition, activity model and reference state beside the pure-solvent property. Identify whether the requested result is solvent partial pressure or total vapor pressure. For nonideal vapor, retain the fugacity treatment appropriate to the state rather than silently extending the simple partial-pressure form.

These assigned values are not solvent data. The illustration selects no solvent formulation, predicts no boiling temperature and does not interpret ionic activity or pH.

Does the mixture equilibrium use the solvent activity appropriate to the reported vapor-pressure state?

This blank worksheet is for your own project. It contains no H M machine trial result.

Customer Questions

Primary References

These references support the technical principles discussed in this guide. The worked examples and review questions are educational.

• IUPAC's Raoult-law formulation
• Reference from hmpharmamachines.com