In the following acid/base pairs find the ones that make buffer (there can be more than one – find all!). Br?

The second, third, and fifth options will produce buffers. Remember, solutions of a WEAK acid or base and its conjugate base or acid will produce buffer. Strong acids do not produce buffers.

NH4+ is a weak acid. Its conjugate base (remove one H+) is NH3. They are mixed, so this solution will be a buffer.

H2S is a weak acid. Its conjugate base is HS-. Thus, the solution will produce a buffer.

CH3COOH (acetic acid) is a weak acid. CH3COO- is its conjugate base, so the solution is a buffer.

HCl and HNO3 are strong acids, so they do not produce buffers (because they fully ionize).

Remember, a buffer is based on a stable, measurable equilibrium of the reaction:

HA ==> A- + H+ where HA is an arbitrary weak acid, A- is the conjugate base of HA, and H+ is hydrogen ions. Strong acids leave negligible HA, so there is no buffer under normal conditions. Weak acids leave significant amounts of HA, so a buffer forms when A- is also added. Here's how a buffer works:

An equilibrium forms between the HA and the A- and H+ so that changes in pH are resisted. When an acid is added to a buffer, the concentration of H+ is increased, and more HA forms to compensate. This reduces the H+ concentration in the process. When a base that does not contain A- is added to the buffer, the concentration of H+ is reduced (acid-base reaction, yielding water), so more HA ionizes to compensate, bringing the concentration of H+ back up and also increasing the concentration of A-. Thus, the buffer can resist changes in pH when either acids or bases are added.

HCl, NH4, and the last two definitely will

H2S and HS- might not, polyprotic acid

CHE-10044 class test I don't know the answers either btw.