Can we determine apriori the volume of solvent in an electrolyte
You can calculate it using partial molar volumes.

The partial molar volume is defined as the differential change in volume with respect to change in mole fraction, and is determined experimentally. From the definition of partial molar properties, you can calculate the amount of one component needed to achieve a certain volume, if you know all of the partial molar volumes and pure component molar volumes.

There may be some complications to that for this system because you are talking about mixing a solid-phase ionic compound with water, instead of two liquids directly.

In practice, it is much easier to just use a volumetric flask so that you know the total volume is 1 L. If you measure the amount of water you actually used you will find that it is very close to 1 L.