ISOPIESTIC DETERMINATION OF THE OSMOTIC AND ACTIVITY-COEFFICIENTS OF (ZH(2)SO(4)-Z)MGSO4)(AQ) AT THE TEMPERATURE T=298.15 K .1. RESULTS FORZ=(0.85811, 0.71539, AND 0.57353)((1)

Isopiestic vapor-pressure measurements were made for {zH(2)SO(4) + (1 - z)MgSO4}(aq) with H2SO4 molality fractions of z = (0.85811, 0.71539, and 0.57353) at the temperature 298.15 K. Measurements extend from to tal molalities m(T) of (0.12548, 0.13563, and 0.14741) mol.kg(-1) to t he supersaturated molalities m(T) = (12.049(6), 11.011(2), and 7.2059( 5)) mol.kg(-1), respectively. For z =(0.71539 and 0.57353) these highe st molalities correspond to the greatest degrees of supersaturation th at could be achieved during isothermal removal of the solvent. The cor responding water activity ranges are 0.9958 greater than or equal to a (w) greater than or equal to 0.2758, 0.9958 greater than or equal to a (w) greater than or equal to 0.3312, and 0.9958 greater than or equal to a(w) greater than or equal to 0.5581 for z = (0.85811, 0.71539, and 0.57353), respectively. These values of z were chosen to be separated approximately equally by 0.142 (Delta z approximate to 1/7). The wate r activity curves were also found to be nearly equally spaced from eac h other, as was the z = 0.85811 curve from that for H2SO4(aq) (z = 1). Given that both H2SO4(aq) and MgSO4(aq) are extensively associated el ectrolytes, this regularity in the mixing behavior was not anticipated . A well-defined crossover point was found at which the sums of the st oichiometric ionic molalities were equal at isopiestic equilibrium for z = (1,0.85811, 0.71539, and 0.57353), as were the water activities a nd stoichiometric osmotic coefficients of these solutions. This occurr ed at Sigma(i) nu(i)m(i) = (5.9632 +/- 0.0036) mol.kg(-1), where nu(i) is the stoichiometric ionization number of electrolyte i. (C) 1997 Ac ademic Press Limited.