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The hydronium ion is present in the aqueous environment. Its molarity is related to the molarity of the hydroxide ion by the ionic product of water:
$[OH^-][H_3O^+]$ $=$ $10^{-14}\frac{mol}{L} $
In acid medium: $[H_3O^+]\gt10^{-7}\frac{mol}{L}$, so $[OH^-]\lt10^{-7}\frac{mol}{L}$ In neutral medium: $[H_3O^+]=10^{-7}\frac{mol}{L}$, so $[OH^-]=10^{-7}\frac{mol}{L}$ In basic medium: $[H_3O^+]\lt10^{-7}\frac{mol}{L}$, so $[OH^-]\gt10^{-7}\frac{mol}{L}$