A systematic study on aqueous solutions of uni- and polyvalent salts of a strong acid type of polyelectrolyte with very low and medium charge density was carried out after application of a special purification method to minimize the effect of ionic contamination. A very effective gel-deswelling technique was used for the measurement of relative activity of water in a wide range of concentration with reproducibility of +/-3 x 10(-6) -2 x 10(-5) in the range -5 x 10(-6) > ln a(1) > -2 x 10(-3) at 298 +/- 0.01 K. The analysis of data has shown that the lattice theory of polymer solution could be successfully applied to polyelectrolyte solutions introducing only a slight modification. The osmotic properties of such systems, the degree of dissociation, and related quantities will be discussed in the paper. A new method for separation of ionic effect will also be given.