stability constant- can someone please check if this is correct

Kstab = [CoCl4^2-] / [[Co(H2O)6]^2+] [Cl-]

Note that water has been left out of the expression for kstab. This is common practice because all the species are dissolved in water, which is in large excess and its concentration is virtually constant.

Why is water in a large excess?

And also why is the concentration always constant? What makes this happen in water and not in other chemicals?

Water is a common solvent for making up solutions (hence aqueous this, aqueous that, etc)

density of water is 1 g per cm cube => 1 dm cube of water weighs 1000 g; concentration of water = mole of water/1 dm 3 = 1000/18 = roughly 50 M

two reactants (aqueous) added together; if both are made up in water (aqueous), then does the amount of water/concentration of water change?

Water is a common solvent for making up solutions (hence aqueous this, aqueous that, etc)

density of water is 1 g per cm cube => 1 dm cube of water weighs 1000 g; concentration of water = mole of water/1 dm 3 = 1000/18 = roughly 50 M

two reactants (aqueous) added together; if both are made up in water (aqueous), then does the amount of water/concentration of water change?

Actually I think I understand. The water doesn't react with either the reactants or products so the concentration remains the same. Is this correct?

water is the solvent here; is it reacting?

You're mean.