Is #NaHCO_3# an acid or base?
This molecule is a salt. You have to see what happens after the salt dissociates in water and what contributes to the pH of the solution.
The Na+ comes from NaOH, the HCO3- comes from carbonic acid, H2CO3. NaOH is a strong base that completely dissociates in water. The product, Na+, is inert and really stable. H2CO3 is a weak acid and does not completely dissociate. The product, HCO3-, is not stable and can react again with water to contribute to the pH.
HCO3- will act as a base by plucking off the proton from H2O, and forming a basic solution.
Strong acids and bases have a high Ka and Kb value and so their equilibriums lie far towards the product side. Equilibrium favors the most stable product and that's why virtually all strong acid and base conjugates are inert.
Acting as an acid
Acting as a base