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Brönsted-Lowry theory doesn't go against but adds to the Arrhenius theory on bases and acids. $\ce{NaOH}$ is an Arrhenius base because it releases hydroxide ions in water solution. NaOH is also a Brönsted-Lowry base because it dissociates in water and its hydroxide ion reacts with water(which acts as an acid).

$\ce{NaOH = Na+ + OH-}$

So if the "actual" Brönsted-Lowry base in NaOH is the hydroxide ion then is the conjugate acid here water? Is the conjugate acid of NaOH, water?

My book on the other hand, says that "$\ce{Na+}$ is the conjugate weak acid to NaOH which is a strong base. So when NaCl is dissolved in water, $\ce{Na+}$ will not react with water at all" . But how can $\ce{Na+}$ be an acid when it doesn't have any hydrogen to give?

So which one is the conjugate acid, $\ce{H2O}$ or $\ce{Na+}$ and how can $\ce{Na+}$ be an acid at all?

Can $\ce{Na+}$ be a conjugate lewis acid but not be a conjugate Brönsted-Lowry acid?

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The conjugate acid to the Brønsted-Lowry base $\ce{OH−}$ is $\ce{H2O}$, just added a proton to $\ce{OH−}$.

$$\ce{B(aq) + H+(aq) <=> BH+(aq)}$$

$$\ce{OH-(aq) + H+(aq) <=> H2O(l)}$$

where $\ce{B} = \ce{OH-}$

OTOH, $\ce{H2O}$ is also the conjugate base to $\ce{H3O+}$.

$\ce{Na+(aq)}$ is an extremely weak Lewis acid. But in a gaseous phase, as a naked ion, it reacts with Lewis base $\ce{Cl-}$, forming an ionic pair $\ce{Na+Cl-}$.

$\ce{Na+}$ could be formally considered as a conjugate Arrhenius acid to $\ce{NaOH}$, but I am not sure if the concept of conjugate pairs is ever used in the Arrhenius theory.

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  • $\begingroup$ In Arrhenius there's no conjugate acid, but $\ce{Na(H2O)+}$ complex can be considered one in B-L. $\endgroup$
    – Mithoron
    Apr 6, 2023 at 17:05

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