Which of the following pairs of substances, when mixed in any proportion you wish, can be used to prepare a buffer solution? (Select all that apply.)

NaCN and HCN
NaCN and NaOH
HCN and NaOH
HCl and NaCN
HCl and NaOH

and Which of the following gives a buffer solution when equal volumes of the two solutions are mixed? (Select all that apply.)

0.10 M NH3 and 0.10 M HCl
0.20 M NH4Cl and 0.10 M NaOH
0.20 M NH3 and 0.10 M HCl
0.10 M NH4Cl and 0.10 M NH3

I know the second one is at least .20M NH3 and .10M HCl but idk what else

NaCN and NaOH

HCN and NaOH
HCl and NaCN
HCl and NaOH

To determine which pairs of substances can be used to prepare a buffer solution, you need to identify the characteristics of a buffer solution. A buffer solution is a solution that resists changes in pH when an acid or base is added. It typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid.

Let's go through the options and identify the potential buffer pairs:

1. NaCN and HCN:
- NaCN is the salt of a weak base, and HCN is a weak acid. This pair can be used to prepare a buffer solution.

2. NaCN and NaOH:
- NaCN is the salt of a weak base, and NaOH is a strong base. This pair cannot be used to prepare a buffer solution.

3. HCN and NaOH:
- HCN is a weak acid, and NaOH is a strong base. This pair cannot be used to prepare a buffer solution.

4. HCl and NaCN:
- HCl is a strong acid, and NaCN is the salt of a weak acid. This pair cannot be used to prepare a buffer solution.

5. HCl and NaOH:
- HCl is a strong acid, and NaOH is a strong base. This pair cannot be used to prepare a buffer solution.

Based on the analysis, the pairs of substances that can be used to prepare a buffer solution are:
- NaCN and HCN

For the second set of options, you need to identify the pairs that can form a buffer solution when equal volumes are mixed. For a buffer solution to form, you need to have a weak acid and its conjugate base or a weak base and its conjugate acid in roughly equal concentrations.

Let's evaluate the options:

1. 0.10 M NH3 and 0.10 M HCl:
- NH3 is a weak base, and HCl is a strong acid. This pair cannot form a buffer solution.

2. 0.20 M NH4Cl and 0.10 M NaOH:
- NH4Cl is the salt of a weak acid, and NaOH is a strong base. This pair cannot form a buffer solution.

3. 0.20 M NH3 and 0.10 M HCl:
- NH3 is a weak base, and HCl is a strong acid. This pair cannot form a buffer solution.

4. 0.10 M NH4Cl and 0.10 M NH3:
- NH3 is a weak base, and NH4Cl is the salt of a weak acid. This pair can form a buffer solution when equal volumes are mixed.

Based on the analysis, the pair that can form a buffer solution when equal volumes are mixed is:
- 0.10 M NH4Cl and 0.10 M NH3