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Because the conversion of H2O,in pure water, into H3O+ and OH- is an autoprolytic reaction, this necessitates that the concentrations of OH- and H3O remain the same. In other words, for H3O+ to be formed, a proton must dissociate from an H2O molecule and bind to another H2O molecule, thus creating OH- (in the former H2O molecule) and H3O+ (in the latter).
Kw refers to the concentration of water molecules and since water is a neutral substance on the pH scale, the concentrations of [H3O+] and [OH-] are the same because when you calculate the pH given these numbers, you will get 7, which is right in the middle of the pH scale
Also, you must keep in mind that we do not begin with any H3O+ or OH- in the solution. It starts out as 100% H2O. After that, as previously stated, note that H3O+ cannot be created without the formation of OH- in a pure water solution.
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