What is the principal inorganic solvent in cells?

The principal inorganic solvent in (living) cells is water.

What is the difference between an organic and inorganic compound?

An organic compound is one whose molecular structure contains carbons and usually almost always hydrogen. Examples of such are sugars, proteins, lipids and nucleic acids e. g. DNA.

An inorganic compound is one which does not contain carbon in its molecular structure e. g. salts and metals.

Water's formula has two hydrogen atoms and one oxygen atom but no carbon atom at all and therefore cannot be an organic compound but inorganic.

Water makes up 60 - 75 % of the cell mass and is the best and most efficient solvent ever.