| Isopiestic measurements were known to be one of the most accurate methods in determining thermodynamic properties for aqueous solutions and had been used mainly for binary and ternary unsaturated solutions. This method for various multi-component unsaturated solutions could be used directly to check the validity of partial ideal solution model. The experimental data obtained by this method had filled the information gaps in the literature about the osmotic coefficients of unsaturated multi-component solutions and fit the partial ideal solution model within experimental uncertainties.To NaCl (aq) or CaCl2 (aq) for the reference,systematic isopiestic measurements for thermodynamic propeities had been carried out at the temperature 298.15K for the mixed rare earth and non rare earth nitrate aqueous solutions including{H2O(A)+La(NO3)3(B)+NaNO3 (C)},{H2O(A)+Y(NO3)3(B)+NaNO3(C)},{H2O(A)+Y(NO3)3(B)+K(NO3)2(C)},{H2O(A)+Y( NO3)3(B)+Ca(NO3)2(C)},{H2O(A)+Y(NO3)3(B)+Mg(NO3)2(C)}, The results shown that the first,second and the third unsaturated ternary aqueous solutions system didn't fellow the linear concentration rules little and they deviated from the partial ideal solution model, but the latter unsaturated aqueous solutions fellow the linear concentration rules at constant activity of one component water.It could be seen through the first, secondand the third group experiments results that the deviated value increased with molality increasing. And in the same line of equal water activity, the deviated value was largest in the middle region with mB/mC≈1. If mB/mC was much larger or smaller, the deviated value was very small. So in the unsaturated termary rare earth nitrate aqueous solutions we had studied, there were different structures and chemical properties between unsaturated solutes La(NO3)3 and NaNO3, Y(NO3)3 and NaNO3, KNO3. The interaction energy among La(NO3)3 and NaNO3, Y(NO3)3 and NaNO3, KNO3 couldn't be neglected.
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