When NaHCO3 is added to water should give an H$^+$ ion in water and this should make it acidic. Why is it a basic salt? Does it have something to do with H2CO3 being a weak acid?


marked as duplicate by Todd Minehardt, Loong, Jan, orthocresol, bon Apr 24 '16 at 19:15

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  • $\begingroup$ The pKa for $\ce{NaHCO3}$ is readily available using Google $\endgroup$ – MaxW Apr 24 '16 at 5:16

The thing is that $\ce{HCO3-}$ can act as an acid and a base. These substances are called amphiprotic (other examples include $\ce{HSO4-}$ and $\ce{H2PO4-}$). So it can donate a proton to water to form $\ce{H3O+}$ but it can also accept a proton from water to form $\ce{OH-}$. So really, the question is which process occurs to a greater extent?.

You can't answer that question by simply looking at the molecule. Instead we need to know the $\mathrm{K_a}$ values for $\ce{H2CO3-}$ and $\ce{HCO3-}$. By comparing these values and doing a few calculations we can determine whether $\ce{HCO3-}$ is basic or acidic. It turns out $\ce{HCO3-}$ is actually basic.

However I am assume that equilibrium constants are probably above the level that you are learning at. So this is just one of these things which you should just know. For example you should know that despite $\ce{HCO3-}$ being basic, $\ce{HSO4-}$ and $\ce{H2PO4-}$ are actually acidic.


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