Question

Which of the following solutions would make a good buffer system? (Check all that apply.) A. A solution that is 0.10 M NH3 and 0.10 M NH4Cl B. A solution that is 0.10 M HCN and 0.10 M NaF C. A solution that is 0.10 M HCN and 0.10 M LiCN D. A solution that is 0.10 M HF and 0.10 M NaF

Answer 1

Good buffer systems are:

A) NH3 + NH4Cl

C) HCN + LiCN

D) HF + NaF

Step-by-step 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

or

2) Weak alkaly + salt

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:

A) 0.10 M NH3 and 0.10 M NH4Cl. It is a buffer because NH3 (ammonia) is a weak alkaly and NH4Cl is a salt coming from NH3.

Buffer component reactions:

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

Reaction salt in water: NH4Cl ---> NH4+ + Cl-

NH4+ is the cation of the weak alkaly so it must be part of the salt in the buffer system. Then NH4Cl is a salt from NH3.

C) 0.10 M HCN and 0.10 M LiCN. It is a buffer because HCN is a weak acid and LiCN is a salt which is coming from HCN.

Buffer component reactions:

Reaction weak acid: HCN + H2O <-----> H3O+ + CN-

Reaction salt in water: LiCN --> Li+ + CN-

CN- is the anion of the acid, so it must be part of the salt in the buffer system. Then LiCN is a salt from HCN.

D) 0.10 M HF and 0.10 M NaF. It is a buffer because HF is a weak acid and NaF is a salt which is coming from 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 (HF), so it must be part of the salt in th buffer systema. Then NaF is a salt coming from HF.

However option B, it is not a buffer, because it is a mixture of 0.10 M HCN and 0.10 M NaF. Salt is not coming from the weak acid.

Reaction weak acid: HCN + H2O <-----> H3O+ + CN- (anion of the acid is CN-)

Rection salt in water: NaF --> Na+ + F- (anion in the salt is F-, not CN-)

Anion of the acid is CN- and the anion in the salt is F- so it is not a salt coming from the weak acid. Then option B it is not a buffer system.

Answer 2

A solution that is 0.10 M HCN and 0.10 M LiCN

. A solution that is 0.10 M NH3 and 0.10 M NH4Cl

Step-by-step explanation:

A buffer consists of a weak acid and its conjugate base counterpart. HCN is a weak acid and the salt LiCN contains its counterpart conjugate base which is the cyanide ion. A buffer maintains the pH by guarding against changes in acidity or alkalinity of the solution.

A solution of ammonium chloride and ammonia will also act as a basic buffer. A buffer may also contain a weak base and its conjugate acid.