EXPERIMENTAL-DETERMINATION OF THE HEAT OF FUSION AND CALCULATION OF THE DISSOCIATION DEGREE AND DISSOCIATION ENTHALPY OF K3FMOO4

The molar enthalpy of fusion Delta(fus)H(m) of KF . K2MoO4 at the temp erature of fusion, 1025 K, was measured in the high-temperature Setara m HTC 1800 K calorimeter as 58 +/- 2 kJ mol(-1). This quantity, togeth er with the known phase diagram of the system KF-K2MoO4 and the compos ition dependence of the enthalpy of mixing at 1273 K, enabled both the dissociation enthalpy Delta(dis)H and the dissociation degree a of th e compound KF . K2MoO4 to be calculated at the temperature of its fusi on. The Le Chatelier-Shreder equation, derived without taking hypothet ical equilibrium states into account, was also used. The most reliable Values of the quantities are those obtained by an iterative procedure using a ''quasi-athermal'' solution model. Their values are Delta(dis ,+)H(1, 1025 K) = -1.2 kJ mol(-1) and alpha(+)(KF . K2MoO4, 1025 K) = 0.51.