Final answer:
Out of the given mixtures, only the mixture of HCHO2 (formic acid) and NaCHO2 (sodium formate) makes a buffer because it consists of a weak acid and its conjugate base.
Step-by-step explanation:
When determining whether or not each mixture is a buffer, we are looking for a combination of a weak acid and its conjugate base or a weak base and its conjugate acid. Buffers resist changes in pH when small amounts of acid or base are added due to the presence of both an acidic species and its conjugate base, or a basic species and its conjugate acid in the solution.
a. KOH is a strong base and NH3 (ammonia) is a weak base. This combination does not make a buffer because it lacks an acidic component that is the conjugate of a weak base.
b. HCl is a strong acid and HBr is another strong acid. This combination also does not make a buffer as it only consists of strong acids without their conjugate bases.
c. HCHO2 is formic acid, a weak acid, and NaCHO2 is the sodium salt of the formate ion, which is the conjugate base of formic acid. This combination does create a buffer because it has both the weak acid and its conjugate base.
d. HBr is a strong acid and NaCl is a salt resulting from a strong acid (HCl) and a strong base (NaOH). Hence, this combination does not make a buffer as it includes a strong acid and a salt that does not contain the conjugate of a weak acid or base.