How would you prepare 250ml of 0.2M sodium phosphate buffer of PH 6.4 if the following chemicals are available :dibasic sodium phosphate,monobasic sodium phosphate and 0.129M solution of NaOH. Assume that the pka1 pka2 pka3 of phosphoric acid are 2.12, 6.8 and 12.3 respectively. Molecular weights of dibasic and monobasic sodium phosphate salts are 142 and 138 respectively. Please this is an urgent question

Use Henderson-Hasselbalch equation and solve for (base)/(acid) ratio. That is equation 1. Use pKa2. Then set up

(base) + (acid) = 0.2 for equation 2. Solve those simultaneously for (acid) and (base)
Convert (acid) to grams NaH2PO4
Convert (base) to grams Na2HPO4.
Remember this will be grams in 250 mL.
NOTE: The molecular weights are not right. Better check those again.

reserch

To prepare 250 ml of a 0.2M sodium phosphate buffer with a pH of 6.4, you will need to use a combination of dibasic sodium phosphate (Na2HPO4), monobasic sodium phosphate (NaH2PO4), and 0.129M NaOH solution. Here are the step-by-step instructions:

1. Calculate the amount of dibasic sodium phosphate needed:
- Convert the desired molarity (0.2M) and volume (250 ml) to moles:
Moles of Na2HPO4 = 0.2 mol/L * 0.25 L = 0.05 moles
- Calculate the mass of dibasic sodium phosphate using its molecular weight (142 g/mol):
Mass of Na2HPO4 = 0.05 moles * 142 g/mol = 7.1 g

2. Calculate the amount of monobasic sodium phosphate needed:
- The buffer solution must have a pH of 6.4, which is closer to pKa2 (6.8) of phosphoric acid. Therefore, we need to alter the ratio of Na2HPO4 and NaH2PO4 to give a pH closer to pKa2.
- Use the Henderson-Hasselbalch equation to determine the required ratio (A:B) of Na2HPO4:NaH2PO4:
pH = pKa + log10 (A/B)
6.4 = 6.8 + log10 (A/B)
log10 (A/B) = -0.4
A/B = 0.4
- Since A + B = 0.05 moles (the total moles of sodium phosphate), we can solve for A and B using simultaneous equations:
A + B = 0.05
A/B = 0.4
- Solving these equations, we find A = 0.032 moles and B = 0.018 moles.
- Calculate the mass of monobasic sodium phosphate using its molecular weight (138 g/mol):
Mass of NaH2PO4 = 0.018 moles * 138 g/mol = 2.484 g

3. Prepare the buffer solution:
- Dissolve 7.1 g of dibasic sodium phosphate (Na2HPO4) in approximately 200 ml of distilled water. Stir until completely dissolved.
- Add 2.484 g of monobasic sodium phosphate (NaH2PO4) to the solution and continue stirring until fully dissolved.
- Adjust the pH to 6.4 using the 0.129M NaOH solution. Add the NaOH solution gradually while monitoring the pH with a pH meter until the desired pH is reached.
- Finally, bring the total volume up to 250 ml with distilled water.
- The resulting solution will be a 0.2M sodium phosphate buffer with a pH of 6.4.

Note: It is important to handle chemicals safely and follow proper laboratory procedures when preparing solutions.

To prepare a 250ml of 0.2M sodium phosphate buffer with a pH of 6.4, you'll need to use the available chemicals: dibasic sodium phosphate, monobasic sodium phosphate, and a 0.129M solution of NaOH. Here are the steps to follow:

Step 1: Calculate the amount of dibasic sodium phosphate needed.
The molecular weight of dibasic sodium phosphate (Na2HPO4) is 142 g/mol. To calculate the amount needed, use the formula:
grams of dibasic sodium phosphate = (moles of dibasic sodium phosphate) × (molecular weight of Na2HPO4)

Since you want a 0.2M solution and have 250ml of final volume, the moles of dibasic sodium phosphate can be calculated using the formula:
moles of dibasic sodium phosphate = (Molarity) × (Volume in liters)
moles of dibasic sodium phosphate = (0.2 mol/L) × (0.25 L)

Now, plug this value into the grams calculation:
grams of dibasic sodium phosphate = (0.2 mol/L) × (0.25 L) × (142 g/mol)

Step 2: Calculate the amount of monobasic sodium phosphate needed.
The molecular weight of monobasic sodium phosphate (NaH2PO4) is 138 g/mol. Use the same method described above to calculate the amount needed:
grams of monobasic sodium phosphate = (0.2 mol/L) × (0.25 L) × (138 g/mol)

Step 3: Determine the pH adjustment using the NaOH solution.
Since the pH of the buffer needs to be 6.4, you'll need to determine the amount of NaOH required to adjust the pH.

First, calculate the pKa value closest to the target pH. In this case, it is pKa2 = 6.8.

Now, use the Henderson-Hasselbalch equation to calculate the ratio of [HPO4^2-] to [H2PO4^-] needed for pH adjustment:
pH = pKa2 + log([HPO4^2-] / [H2PO4^-])

Plug in the known values:
6.4 = 6.8 + log([HPO4^2-] / [H2PO4^-])

Rearrange the equation to solve for [HPO4^2-] / [H2PO4^-]:
[HPO4^2-] / [H2PO4^-] = 10^(pH - pKa2)

Calculate the ratio:
[HPO4^2-] / [H2PO4^-] = 10^(6.4 - 6.8)

Step 4: Calculate the amount of NaOH needed to adjust the pH.
To calculate the amount of NaOH required, you need to determine the millimoles (mmol) of H2PO4^- and HPO4^2- from steps 1 and 2.

mmol H2PO4^- = moles of monobasic sodium phosphate × 1000 (to convert to mmol)
mmol HPO4^2- = moles of dibasic sodium phosphate × 1000 (to convert to mmol)

Now, calculate the amount of NaOH needed using the molar ratio:
grams of NaOH = (mmol H2PO4^- - mmol HPO4^2-) × (40 g/mol)

Step 5: Prepare the buffer solution.
Using a balance, measure the calculated amounts of dibasic sodium phosphate and monobasic sodium phosphate and add them to a container. Add distilled water and mix to dissolve the salts completely.

Next, measure the amount of NaOH calculated in Step 4 and add it to the buffer solution while monitoring the pH using a pH meter or pH paper. Stir the solution until the desired pH of 6.4 is achieved.

Finally, adjust the final volume to 250ml by adding more distilled water if necessary.

Please note that it's important to double-check all calculations and ensure the accurate measurement of chemicals to prepare the desired buffer solution.