[T]

Hello,

I have recently learnt you can find the molarity of H+ if you have the molarity of OH- through the ionic product of water.

However I don't understand why can we use the ionic product of water? The ionic product of water is derived from the equation:

2H₂0  H₃0⁺ + OH^-

From this equation the equilibrium constant can be found and this is the ionic product of water. However how is it applied in the case of an equation such as this as the equations are different?

HCl + H₂0  Cl^- + H₃0⁺

I tried to explain it by saying that the Cl^- are spectator ions so therefore they are eliminated. But that gives you this.

H⁺ + H₂0  H₃0⁺

Can someone please explain this to me? Thanks

[Borek]

Nothing stops you from having several equilibria present in the solution at the same time.

[mjc123]

In a system at equilibrium, all relevant equilibria must be satisfied simultaneously. So for example, in aqueous solution, the equilibrium
2H₂O  H₃O⁺ + OH^-
is always there, whatever else is also present. (by the way, try and distinguish Os from zeros) And it is always true, whatever else is going on, that [H₃O⁺][OH^-] = 10^-14M (adjust for temperature if necessary). So if you have, say, 0.1M HCl, which is a strong acid and completely dissociated, you have 0.1M H₃O⁺, and therefore 10^-13M OH^-. (Weak acids and bases are a bit more complicated.)

By the way, HCl + H₂O  H₃O⁺ + Cl^-
is not equivalent to H⁺ + H₂O  H₃O⁺
You don't get free protons in water. You cannot treat Cl^- here as a spectator ion, because it is not present in undissociated HCl, which is a covalent molecule. The equation describes the dissociation of HCl into ions in water.