Calculation of vapour pressures from effusion measurements for dissociated vapours

Abstract

For a substance dissociating in the vapour according to the scheme X(c)⇌ν_AA +ν_BB +…+ν_RR, the following expressions are derived relating P*, the true equilibrium vapour pressure, P, the actual pressure at the steady state during a Knudsen effusion experiment, and P′, the nominal pressure derived from weight loss measurements on the assumption that no dissociation takes place: P*/P=νΠM^ν_i/2ν_i/Σν_iM^½_i, P*/P′=νΠM^ν_i/2ν_i/M^½_X(ν=Σν_i) These results differ from those derived by Pashinkin, whose assumption that the vapour within the cell retains the original stoichiometry (while the numbers of effusing molecules alter their proportions) cannot be valid at the steady state. Use of a mean “molecular weight”, M_X/ν, in calculating the nominal pressure from weight loss results gives a result very near to P, which is also measured directly by the torsion method; but conversion to P* is still necessary in principle if the true equilibrium pressure is required. Incomplete dissociation is also considered. It is assumed throughout that chemical and phase equlibria are effectively achieved in the steady state.