According to the Brønsted-Lowry theory, which of the following would you expect to act as an acid? Which as a base? (a) \(\mathrm{CHO}_{2}^{-}\) (b) \(\mathrm{NH}_{4}^{+}\) (c) \(\mathrm{HSO}_{3}^{-}\)

We have to see if the \(\mathrm{CHO}_{2}^{-}\) can donate or accept a proton. Looking at the structure of \(\mathrm{CHO}_{2}^{-}\) (formate ion), there is an oxygen with a negative charge and a hydrogen bonded to it. Oxygen, being electronegative, can attract a proton and form a bond (O-H), giving us a molecule of formic acid (\(\mathrm{HCO}_{2}\mathrm{H}\)). Therefore, \(\mathrm{CHO}_{2}^{-}\) can act as a Brønsted-Lowry base.

Now let's analyze the \(\mathrm{NH}_{4}^{+}\) ion (ammonium ion). The ammonium ion has a positive charge and already has four hydrogen atoms bonded to the nitrogen. If the \(\mathrm{NH}_{4}^{+}\) were to donate one of its hydrogen atoms as a proton, it would turn into ammonia (\(\mathrm{NH}_{3}\)) which is a neutral molecule. Therefore, \(\mathrm{NH}_{4}^{+}\) can act as a Brønsted-Lowry acid.

Lastly, let's consider the \(\mathrm{HSO}_{3}^{-}\) ion (hydrogen sulfite ion). This ion has a sulfur atom bonded to three oxygen atoms and one hydrogen atom. One of the oxygen atoms has a negative charge and a hydrogen atom bonded to it. The hydrogen atom can be donated as a proton, resulting in a sulfite ion (\(\mathrm{SO}_{3}^{2-}\)). Therefore, the \(\mathrm{HSO}_{3}^{-}\) can act as a Brønsted-Lowry acid. However, since there is also a negatively charged oxygen within the structure, the hydrogen sulfite ion can also accept a proton, resulting in a molecule of sulfurous acid (\(\mathrm{H}_{2}\mathrm{SO}_{3}\)). Thus, \(\mathrm{HSO}_{3}^{-}\) can act as both a Brønsted-Lowry acid and a base, making it an amphoteric species.