When thinking about Henry’s law one associates it with the solubility constant for gases in liquids at thermodynamic
equilibrium. A practical application is checking gas sensors for specificity [1-3]. A gas sensor is for example the main part
of an evidential breath analyser (EBA), a device to determine breath alcohol content of a person. Nowadays the empirical
Dubowski formula, which is recommended officially, is solely suitable to convert aqueous ethanol concentration into
ethanol concentration in air. Therefore, it is necessary to establish a theoretical based calculation method to enable
synthesizing a humidified gas phase with any solute in a predefined concentration by dissolving the solute in water at the
right concentration. This note will explain how to use Henry’s law to derive an equation to perform this calculation for an
arbitrtary solute. This equation, derived from Henry’s law has the same predictive power compared to Dubowski formula.