Final answer:
The correct substance to convert a 0.30 M nitrous acid solution into a buffer is 0.31 mol NaNO2, as it provides the necessary conjugate base, nitrite ion (NO2-), for the nitrous acid buffer system. Other options are not suitable for establishing the appropriate buffering pair.
Step-by-step explanation:
A buffer solution is a solution that can resist pH changes upon the addition of an acid or a base. In this case, an aqueous solution of 0.30 M nitrous acid can be transformed into a buffer by adding a substance that provides its conjugate base, nitrite (NO2-).
Option A, 0.31 mol NaNO2, is the correct choice as NaNO2 is the sodium salt of the nitrite ion, which will provide the necessary conjugate base (NO2-) to pair with nitrous acid (HNO2) to form a buffer. Adding NaNO2 will not significantly change the volume of the solution. This is because the salt will dissociate to Na+ and NO2- ions, which do not significantly contribute to the volume.
Other options, such as B, C, D, and E, do not provide the necessary conjugate base for nitrous acid. Instead, they either provide ions that are not relevant to the buffering action needed for nitrous acid (like NaBr, which provides Br-), or that would change the pH too drastically (like Ca(OH)2, which is a strong base, and HI, which is a strong acid).