which can act as buffer ?

Options

A. `0.1MNH_(4)OH+0.2MHCl`
B. `HCN+CH_(3)COOK`
C. `40ml,0.1M NaCN+20ml,0.1MHCN`
D. All of the above

Explanation

To determine which combinations can act as a buffer, we need to analyze each option provided in the question. A buffer solution is typically made from a weak acid and its conjugate base or a weak base and its conjugate acid. Let's go through the options step by step.

### Step-by-Step Solution:

1. **Option 1: NH4OH mixed with HCl**
- **Analysis**: NH4OH (ammonium hydroxide) is a weak base, while HCl is a strong acid. When mixed, HCl will dissociate completely into H+ and Cl-. The NH4OH will partially dissociate into NH4+ and OH-. Since HCl is in greater quantity and completely dissociates, the presence of NH4+ (the conjugate acid of NH4OH) will help resist changes in pH when small amounts of acid or base are added.
- **Conclusion**: This combination can act as a buffer.

2. **Option 2: HCN and CH3COOK**
- **Analysis**: HCN (hydrocyanic acid) is a weak acid, and CH3COOK (potassium acetate) is a salt that dissociates into CH3COO- (the conjugate base of acetic acid). The presence of both HCN and CH3COO- allows the solution to resist changes in pH upon the addition of small amounts of acid or base.
- **Conclusion**: This combination can also act as a buffer.

3. **Option 3: 40 mL of 0.1 M NaCN and 0.1 M HCN**
- **Analysis**: NaCN is a salt that dissociates completely into Na+ and CN- (the conjugate base of HCN). In this case, we have both HCN (weak acid) and CN- (conjugate base) in the solution. This combination will effectively resist changes in pH when small amounts of acid or base are added.
- **Conclusion**: This combination can act as a buffer as well.

4. **Final Conclusion**: Since all options provided can form buffer solutions, the correct answer is **"All of the above."**