For preparing a buffer solution of pH = 7.0 which buffer system you will choose?

Options

A. `H_3PO_4,H_2PO_4^-`
B. `H_2PO_4^-,HPO_4^(2-)`
C. `HPO_4^(2-),PO_4^(3-)`
D. `H_3PO_4,PO_4^(3-)`

Answer

To prepare a buffer solution with a pH of 7.0, we need to select an appropriate buffer system. Here’s a step-by-step solution to determine which buffer system to choose:

### Step 1: Understand Buffer Solutions
Buffer solutions are designed to resist changes in pH when small amounts of acid or base are added. They consist of a weak acid and its conjugate base or a weak base and its conjugate acid.

### Step 2: Identify the pKa Values
For the given options, we need to consider the dissociation of phosphoric acid (H3PO4), which is a triprotic acid. The relevant pKa values are:
- pKa1 = 2.15 (for H3PO4 → H2PO4⁻)
- pKa2 = 7.1 (for H2PO4⁻ → HPO4²⁻)
- pKa3 = 12.5 (for HPO4²⁻ → PO4³⁻)

### Step 3: Determine the Suitable pKa for pH 7.0
Since we want a buffer solution with a pH of 7.0, we should choose the pKa value that is closest to 7.0. The pKa2 (7.1) is the closest to our desired pH of 7.0.

### Step 4: Identify the Conjugate Acid-Base Pair
For a buffer solution to maintain a pH of 7.0, we should use the conjugate acid-base pair corresponding to pKa2. This pair is:
- H2PO4⁻ (the conjugate acid)
- HPO4²⁻ (the conjugate base)

### Step 5: Choose the Correct Buffer System
Among the options provided:
1. H3PO4 and H2PO4⁻
2. H2PO4⁻ and HPO4²⁻
3. HPO4²⁻ and PO4³⁻
4. H3PO4 and PO4³⁻

The correct choice is option 2: **H2PO4⁻ and HPO4²⁻**, as this pair will effectively create a buffer solution with a pH around 7.0.

### Final Answer
The buffer system you should choose for preparing a buffer solution of pH = 7.0 is **H2PO4⁻ and HPO4²⁻**.

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