H3PO4 Ka1= 7.5x10^-3 Ka2= 6.2x10^-8 Ka3=4.2x10-^13
Explain how you will make a buffer with a pH of 11.80 using solid Na2HPO4 and either 2.00 M NaOH or 2.00 M HCL. The most concentrated component should have a concentration of 0.50 M, and you need to make 2.50 liters of this buffer.
Where is the explanation for question
To make a buffer with a pH of 11.80 using solid Na2HPO4 and either 2.00 M NaOH or 2.00 M HCl, follow these steps:
Step 1: Determine the desired pH of the buffer. In this case, it is 11.80.
Step 2: Calculate the pOH of the desired pH using the formula: pOH = 14 - pH. In this case, the pOH is 14 - 11.80 = 2.20.
Step 3: Convert the pOH back to pH using the formula: pH = 14 - pOH. In this case, the pH is 14 - 2.20 = 11.80 (as desired).
Step 4: Determine the ratio of the conjugate base to acid needed to achieve the desired pH. This can be done using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]).
Since the acid is H3PO4, let's denote it as HA, and its conjugate base as A-. The pKa values given are for H3PO4, so we'll use the first pKa value (7.5x10^-3) in this case.
Using the Henderson-Hasselbalch equation, we have: 11.80 = -log(7.5x10^-3) + log([A-]/[HA]).
Rearranging the equation, we get: log([A-]/[HA]) = 11.80 + log(7.5x10^-3).
Taking the anti-log of both sides, we get: [A-]/[HA] = 10^(11.80 + log(7.5x10^-3)).
Calculating the value on the right-hand side of the equation, we find: [A-]/[HA] = 10^(11.80) * 7.5x10^-3.
Simplifying further, we get: [A-]/[HA] = 1.6x10^11.
Therefore, the ratio of the conjugate base to acid needed to achieve the desired pH is 1.6x10^11:1.
Step 5: Determine the amount of solid Na2HPO4 needed to achieve a concentration of 0.50 M in a 2.50 L buffer solution. To do this, we use the formula: moles = concentration × volume.
Since we want a 0.50 M solution of Na2HPO4, and we have a 2.50 L solution, we can calculate the moles of Na2HPO4 needed by multiplying the concentration and volume: moles = 0.50 M × 2.50 L.
Therefore, the moles of Na2HPO4 needed is 1.25 moles.
Step 6: Calculate the mass of solid Na2HPO4 needed using its molar mass. The molar mass of Na2HPO4 is the sum of the atomic masses: (2 × atomic mass of Na) + atomic mass of P + (4 × atomic mass of O).
Step 7: Determine whether to use 2.00 M NaOH or 2.00 M HCl to adjust the pH of the buffer. Since we want the most concentrated component to have a concentration of 0.50 M, we would need to dilute the 2.00 M solution.
To calculate the amount needed to dilute either NaOH or HCl, use the formula: C1V1 = C2V2, where C1 and C2 are the initial and final concentrations, and V1 and V2 are the initial and final volumes.
Let's say we decide to use NaOH:
The initial concentration of NaOH is 2.00 M, and the final concentration is 0.50 M.
We already have the final volume needed (2.50 L), so we can calculate the initial volume of NaOH needed:
(2.00 M) × V1 = (0.50 M) × (2.50 L).
Solving for V1, we get: V1 = (0.50 M) × (2.50 L) / (2.00 M).
Therefore, the initial volume of 2.00 M NaOH needed to achieve a final concentration of 0.50 M in a 2.50 L buffer solution is 1.25 L.
Step 8: Prepare the buffer solution:
a) Weigh the calculated mass of solid Na2HPO4 and dissolve it in water to make a 0.50 M solution.
b) Measure the calculated volume of 2.00 M NaOH and add it to the Na2HPO4 solution.
c) Adjust the final volume to 2.50 L using water.
Now, you have successfully made a buffer with a pH of 11.80 using solid Na2HPO4 and 2.00 M NaOH.