Explain the laboratory preparation of one Litre of 1.3M formic acid buffer of pH 3.9 from solid sodium formate (molecular weight =68) and formic acid liquid 30% pure (Density of formic acid = 1.029g/ml, pKa = 3.75)

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To prepare a 1.3M formic acid buffer of pH 3.9 using solid sodium formate and 30% pure formic acid, follow the steps below:

1. Start by determining the amount of formic acid and sodium formate needed to prepare the buffer. We will need to calculate the number of moles of formic acid and sodium formate required.

2. Calculate the moles of formic acid needed:
- Determine the required volume of the buffer. In this case, we need 1 liter of buffer.
- Since the pH is given, you can convert it to the hydrogen ion concentration ([H+]) using the equation: [H+] = 10^(-pH).
- Calculate the moles of formic acid needed using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]).
- Rearrange the equation to find the moles of formic acid: [A-]/[HA] = 10^(pH - pKa).
- With the given pKa (3.75), and since formic acid is a weak acid, we assume that all of it will dissociate to form H+ and HCOO- ions. So, [A-]/[HA] = 1.
- Therefore, we need the moles of formic acid to be equal to the number of moles of H+ required, which is 10^(3.9 - 3.75).

3. Calculate the mass of sodium formate needed:
- Since the molar ratio between sodium formate (NaHCOO) and formic acid (HCOOH) is 1:1, the moles of sodium formate needed will be the same as the moles of formic acid calculated in step 2.

4. Calculate the mass of formic acid:
- Multiply the volume of the buffer (1 liter) by the desired molarity (1.3 moles/liter) to get the number of moles required.
- Multiply this by the molecular weight of formic acid (46 g/mol) to get the required mass of formic acid.

5. Calculate the mass of sodium formate:
- Multiply the moles of sodium formate calculated in step 3 by the molecular weight of sodium formate (68 g/mol) to get the required mass of sodium formate.

6. Prepare the buffer:
- Weigh the calculated masses of sodium formate and formic acid separately using a balance.
- Dissolve the sodium formate in around 800 ml of distilled water in a beaker.
- Slowly add the solid sodium formate to the water while stirring until it fully dissolves.
- Add the required volume of 30% pure formic acid to the sodium formate solution.
- Stir the solution thoroughly to ensure proper mixing.

7. Adjust the pH:
- Measure the pH of the buffer solution using a calibrated pH meter. If the pH is not 3.9, adjust it by adding small amounts of either concentrated formic acid or sodium hydroxide.
- Measure the pH after each addition and continue until the desired pH of 3.9 is reached.

8. Finally, transfer the buffer solution to a 1-liter volumetric flask and make up to the mark with distilled water to achieve a final volume of 1 liter, ensuring accuracy.

By following these steps, you will be able to prepare one liter of a 1.3M formic acid buffer with a pH of 3.9 using solid sodium formate and 30% pure formic acid.