Which of the following aqueous solutions are good buffer systems? . 0.29 M perchloric acid + 0.15 M potassium perchlorate 0.16 M potassium acetate + 0.26 M acetic acid 0.18 M hydrofluoric acid + 0.12 M sodium fluoride 0.31 M hypochlorous acid + 0.28 M potassium hypochlorite 0.35 M ammonium bromide + 0.32 M ammonia

Good buffer systems are:  

Option B)  Potassium acetate (KCH3COO) + acetic acid (CH3COOH).

Option C: Hydrofluoric acid (HF) + sodium fluoride (NaF)

Option D: Hypochlorous acid (HClO) + potassium hypochlorite (KClO):

Option E: Ammonium bromide (NH4Br) + ammonia (NH3)

Explanation:

Buffers consist in a mixture of a weak acid with its salt or a weak alkaly with its salt. All buffer systems are conformed by:

1) Weak acid + salt formed with conjugated base of the acid

or

2) Weak alkaly + salt formed with conjugated acid of the alkaly

It is very important these salts come from the weak acid or weak alkaly. It means, the anion of the acid must be the anion in the salt which is going to be part of the buffer system. On the other hand, the cation of the weak alkaly must be the cation of the salt which is going to form the salt in the buffer system.

Then, when we evaluate all options in this exercise, answers are the following:

Option A) Perchloric acid (HClO4)  + potassium perchlorate (KClO4).

It is not a buffer system because HClO4 is a strong acid. A buffer requires a weak acid or weak alkaly.  KClO4 is a salt formed by the conjugated base of HClO4, but a buffer requires two condition:  Weak acid or alkaly + its salt.

Option B)  Potassium acetate (KCH3COO) + acetic acid (CH3COOH).

This mixture is a buffer because it is formed by a weak acid (acetic acid)  and its salt (KCH3COO is a salt coming from weak acid ---CH3COOH---).

Buffer component reactions:

Reaction weak acid:     CH3COOH + H2O <> H3O+ + CH3COO-

Reaction salt in water:  KCH3COO ---> K+ + CH3COO-

CH3COO- is the anion of the weak acid so it must be part of the salt in the buffer system. Then KCH3COO is a salt from CH3COOH.

Option C: Hydrofluoric acid (HF) + sodium fluoride (NaF)

This mixture is a buffer because it is formed by a weak acid (Hydrofluoric acid)  and its salt (NaF is a salt coming from weak acid ---HF---).

Buffer component reactions:

Reaction weak acid:     HF + H2O <> H3O+ + F-

Reaction salt in water:  NaF ---> Na+ + F-

F- is the anion of the weak acid so it must be part of the salt in the buffer system. Then NaF is a salt from HF.

Option D: Hypochlorous acid (HClO) + potassium hypochlorite (KClO):

It is a buffer because it is formed by a weak acid (hypochlorous acid)  and its salt (KClO is a salt coming from weak acid ---HClO---).

Buffer component reactions:

Reaction weak acid:     HClO + H2O <> H3O+ + ClO-

Reaction salt in water:  KClO ---> K+ + ClO-

ClO- is the anion of the weak acid so it must be part of the salt in the buffer system. Then KClO is a salt from HClO.

Option E: Ammonium bromide (NH4Br) + ammonia (NH3)

The combination of this weak alkaly (NH3) and the salt (ammonium bromide, NH4Br) is a buffer becuase it is formed by a weak compound and its salt.

Buffer component reactions:

Reaction weak alkaly:    NH3 + H2O <> NH4+ + OH-  

Reaction salt in water:    NH4Br ---> NH4+ + OH-

NH4+ is the cation of the weak alkaly so it must be part of the salt in the buffer system. Then NH4Br is a salt from NH3.

Remember a buffer is formed by the combination of two different chemical sustances:  Weak acid or Weak alkaly plus its salt.

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