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I still don't understand why solvent does not count for equilibrium constant. In the example of dissolution of HCl, some water molecules would be added one proton and become a hydronium ion, and both the concentration of water and the concentration of hydronium ions change during the process. Why don't we calculate the equilibrium constant by writing Kc = ([H3O+]*[Cl-])/([H2O]*[HCl])? Even though change in the concentration of water is so small that the net change seems unnoticeable, [H3O+] should be included in the equation because change in the concentration of hydronium ions is relatively significant.
There is no such thing is solvent concentration. When you have a concentration of salts, acid, or any compound, concentration is usually measured in moles per L, or M. The key here is that the moles per liter is moles per liter of water (or whatever your solvent is). It is not possible to have the water concentration decrease or increase, because there is a set amount of moles per liter of water, this will always be the same.
The activity level for a solvent is equal to or very close to being equal to 1, so it doesn't affect the equilibrium constant.
The same rule applies of omitting solids in the equilibrium constant.
The activity level for both solids and liquids are 1
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