Concept:
Basic buffer solution:
- As the name suggests, a basic buffer maintains the basic pH in the solution.
- It is also called an alkaline buffer solution.
- It is prepared by mixing a weak base with a salt of strong acid.
- These buffers have a pH > 7.
- For example - a buffer of NH4OH and NH4Cl.
Explanation:
The basic buffer solution is made by mixing of a weak base with strong acid in such a way to maintain the pH of the solution basic or >7.
⇒ So, a basic buffer has a weak base and a salt of strong acid in the solution.
⇒In 1st solution we have - 50 mL of 0.1 M NaOH + 25 mL of 0.1 M CH3COOH
total moles of NaOH initially 50 × 0.1 = 5 mmol
total moles of CH3COOH initially 25 × 0.1 = 2.5 mmol
2.5 mmole of NaOH neutralize the 2.5 mmol of CH3COOH and formed 2.5 mmole of sodium salt of acetic acid.
2.5 mmol of NaOH remains in the solution.
So, the solution has 2.5 mmol NaOH + 2.5 mmol CH3COONa, which is only a basic solution as NaOH is a strong base, not a weak base.
⇒ In 2nd solution we have - 100 mL of 0.1 M CH3COOH + 100 mL of 0.1 M NaOH
10 mmol of CH3COOH and 10 mmol of NaOH neutralize each other.
Same amount of acid, base, and salt will not form a basic buffer.
⇒ In the 3rd solution we have - 100 mL of 0.1 M HCl + 200 mL of 0.1 M NH4OH
10 mmol of HCl reacts with only 10 mmol of NH4OH and forms 10 mmol of NH4Cl.
10 mmol of NH4OH remains in the solution.
Hence, the solution will have 10 mmol of NH4OH + 10 mmol of NH4Cl.
It makes a basic buffer solution, as the solution has a weak base with a salt of strong acid.
⇒ 4th solution - 100 mL of 0.1 M HCl + 100 mL of 0.1 M NaOH
It is a neutral solution as the same amount of acid and base neutralize each other.
Hence only, option c (100 mL of 0.1 M HCl + 200 mL of 0.1 M NH4OH) makes a basic buffer solution.
∴ the correct answer is option C.