Prepare 250ml of 0.2M phosphate buffer of pH 6.4.starting from Na2HPO4,NaH2PO4,0.129N NaOH solution.

I used a pKa2 for H3PO4 of 7.21. You should use the value in your text/notes.

pH = pKa2 + log (base)/(acid)
6.4 = 7.21 + log b/a
b/a = about 0.15 but you should do all of this more accurately.
Then b = 0.15*a. That's equation 1.
equation 2 is a + b = 0.2 M
Solve these two equation simultaneously for a and b. I obtained values of approx a = 0.17 M and b = about 0.03 M. Again you should do it more accurately.

You want 250 mL; therfore,
millimols acid = 250 x 0.17 = about 43
mmols base = 250 x 0.03 = about 7
mmols buffer = 0.2 x 250 = 50.

So you want to weigh out 50 millimols H2PO4^- (convert to mmols NaH2PO4 and weigh out that much NaH2PO4).
Calculate mL 0.129 M NaOH to add to give you the approx 7 mmols HPO4^- and voila! Place all of this in a 250 mL flask, add water to the mark and mix. I like to check these things by starting with 0.129 M NaOH and the grams NaH2PO4 and see if that actually gives you a pH of 6.4.

To prepare 250 ml of 0.2M phosphate buffer at pH 6.4 using Na2HPO4, NaH2PO4, and a 0.129N NaOH solution, follow these steps:

Step 1: Calculate the molarities required for Na2HPO4 and NaH2PO4:
To prepare a buffer at a particular pH, we need the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])

Given that the desired pH is 6.4, we can assume that pKa = 6.8 for the phosphate buffer.

Let's assume [A-] is x and [HA] is 0.2 - x.

Substituting the values into the Henderson-Hasselbalch equation:
6.4 = 6.8 + log(x/(0.2 - x))

Now solve for x:
x/(0.2 - x) = 10^(6.4 - 6.8)
x/(0.2 - x) = 10^(-0.4)

Cross-multiplying:
x = 0.2 * 10^(-0.4) - 10^(-0.4) * x
1.0234x = 0.2 * 10^(-0.4)
x = (0.2 * 10^(-0.4))/1.0234
x = 0.1949

Therefore, [A-] = 0.1949 and [HA] = 0.2 - 0.1949 = 0.0051

Step 2: Calculate the required amounts of Na2HPO4 and NaH2PO4:
We know the molar mass of Na2HPO4 is 141.96 g/mol, and that the desired concentration is 0.2M.

The amount (in moles) of Na2HPO4 needed can be calculated as follows:
moles Na2HPO4 = 0.2 M * 0.250 L
moles Na2HPO4 = 0.05 mol

Given that the molar mass is 141.96 g/mol, we can calculate the mass of Na2HPO4 required:
mass Na2HPO4 = 0.05 mol * 141.96 g/mol
mass Na2HPO4 = 7.10 g

Similarly, for NaH2PO4:
moles NaH2PO4 = 0.0051 M * 0.250 L
moles NaH2PO4 = 0.001275 mol

mass NaH2PO4 = 0.001275 mol * 120.01 g/mol
mass NaH2PO4 = 0.153 g

Step 3: Calculate the amount of NaOH solution required:
To adjust the pH, we need to add NaOH.

The balanced chemical equation for the reaction is:
NaH2PO4 + NaOH → Na2HPO4 + H2O

The molar ratio between NaH2PO4 and NaOH is 1:1.

moles NaOH = moles NaH2PO4
moles NaOH = 0.001275 mol

To calculate the volume of 0.129N NaOH solution, we need to convert the molarity to moles/liter:
0.129N = 0.129 mol/L

volume NaOH = moles NaOH / molarity
volume NaOH = 0.001275 mol / 0.129 mol/L
volume NaOH = 0.00988 L
volume NaOH = 9.88 mL

Therefore, you would need to add 9.88 mL of 0.129N NaOH solution.

Step 4: Assemble the buffer solution:
a) Dissolve 7.10 g of Na2HPO4 in distilled water to make a final volume of 250 mL.
b) Dissolve 0.153 g of NaH2PO4 in distilled water to make a final volume of 250 mL.
c) Add 9.88 mL of 0.129N NaOH solution to the Na2HPO4 solution.
d) Adjust the final volume to 250 mL by adding distilled water.

Make sure to dissolve each compound separately in distilled water before mixing them together.

By following these steps, you can prepare 250 mL of a 0.2M phosphate buffer at pH 6.4 starting from Na2HPO4, NaH2PO4, and a 0.129N NaOH solution.

To prepare a 250 ml 0.2M phosphate buffer of pH 6.4, we can follow these steps:

1. Determine the required amount of Na2HPO4:
- The molecular weight of Na2HPO4 is 141.96 g/mol.
- We need a final concentration of 0.2M, so using the equation:
(Moles Na2HPO4)/(Volume in liters) = Concentration in M
we can rearrange to find the moles of Na2HPO4:
(0.2 mol/L) * (0.25 L) = 0.05 moles Na2HPO4
- Multiply the moles by the molecular weight to find the required mass of Na2HPO4:
0.05 mol * 141.96 g/mol = 7.098 g Na2HPO4

2. Determine the required amount of NaH2PO4:
- The molecular weight of NaH2PO4 is 119.98 g/mol.
- Since we want the buffer to be at a specific pH, we need to calculate the ratio of Na2HPO4 to NaH2PO4 based on the desired pH.
- For a pH of 6.4, the ratio of Na2HPO4 to NaH2PO4 is 2:1
- Multiply the moles of Na2HPO4 by the ratio to find the moles of NaH2PO4:
0.05 moles Na2HPO4 * (1 mole NaH2PO4 / 2 moles Na2HPO4) = 0.025 moles NaH2PO4
- Multiply the moles by the molecular weight to find the required mass of NaH2PO4:
0.025 mol * 119.98 g/mol = 2.9995 g NaH2PO4

3. Prepare the required volume of buffer:
- The required volume is 250 ml.
- Dissolve the calculated masses of Na2HPO4 (7.098 g) and NaH2PO4 (2.9995 g) in distilled water in a volumetric flask.
- Make sure the solution is thoroughly mixed.

4. Adjust the pH using 0.129N NaOH solution:
- The concentration of NaOH solution is 0.129N. This means it contains 0.129 moles of NaOH in 1 liter.
- Calculate the volume of NaOH solution required to adjust the pH using the equation:
(Volume NaOH solution in liters) = (Moles of NaOH required) / (Concentration of NaOH solution in moles/L)
To adjust the pH to 6.4, we need to use a titration calculator or software to determine the required volume of NaOH solution. This will depend on the specific pKa values of the phosphate buffer system.

5. Once you know the volume of NaOH solution required, carefully add it to the buffer solution while monitoring the pH using a pH meter or indicator. Add the NaOH solution slowly until the desired pH of 6.4 is reached.

Remember to use proper laboratory techniques and precautions while handling chemicals.