HELP!!
Describe how to prepare 800 ml of a 0.100 M potassium acetate buffer, pH = 4.46 (n.b.
pKacetic acid = 4.76). You have glacial acetic acid (17.6 M), a 1.000 M solution of KOH and
a 1.000 M solution of HCl.
To prepare 800 ml of a 0.100 M potassium acetate buffer with a pH of 4.46, you will need glacial acetic acid (17.6 M), a 1.000 M solution of KOH, and a 1.000 M solution of HCl. Follow these steps:
1. Determine the amount of acetic acid (CH3COOH) and potassium acetate (CH3COOK) needed to make the buffer. This calculation requires some stoichiometry and knowledge of the pKa of acetic acid.
The Henderson-Hasselbalch equation for a buffer solution is:
pH = pKa + log([A-]/[HA])
Rearranging the equation, we can solve for [A-]/[HA]:
[A-]/[HA] = 10^(pH-pKa)
Since the pH and pKa are given, you can calculate [A-]/[HA].
[A-]/[HA] = 10^(4.46 - 4.76) = 0.3981
This means for every 0.3981 moles of acetate ion (A-), there will be 1 mole of acetic acid (HA).
2. Calculate the amount of acetic acid (CH3COOH) needed:
The desired molarity is 0.100 M, and you need 800 ml of the final solution.
So, the moles of acetic acid (HA) required = (0.100 M) * (0.800 L) = 0.080 moles
Since the ratio of [A-] to [HA] is 0.3981, you can calculate the moles of acetate ion (A-) needed using this ratio:
Moles of acetate ion (A-) = 0.080 moles * 0.3981 = 0.0318 moles
3. Calculate the amount of potassium acetate (CH3COOK) needed:
Since every mole of acetate ion (A-) requires one mole of potassium acetate (CH3COOK), we need 0.0318 moles of potassium acetate.
4. Calculate the volume of glacial acetic acid (17.6 M) needed:
The molarity of glacial acetic acid is 17.6 M, and we need 0.080 moles of acetic acid.
So, the volume of glacial acetic acid required = (0.080 moles) / (17.6 M) = 0.00455 L = 4.55 ml
5. Calculate the volume of 1.000 M KOH needed to reach pH 4.46:
The pKa of acetic acid is 4.76, and we want to reach pH 4.46, which is slightly more acidic.
To achieve this, we need to add a small amount of a strong base like KOH to react with some of the acetic acid to form acetate ion (A-).
Therefore, the volume of 1.000 M KOH needed can be calculated as follows:
Change in moles of acetic acid (HA) = Moles of acetate ion (A-) * 0.3981
= 0.0318 moles * 0.3981 = 0.0127 moles
The volume of 1.000 M KOH needed = (0.0127 moles) / (1.000 M) = 0.0127 L = 12.7 ml
6. Calculate the volume of 1.000 M HCl needed:
The pH of the final buffer determines how much HCl will be added to adjust the pH.
Since the pH of the buffer is 4.46 and the pKa of acetic acid is 4.76, we know the buffer is slightly acidic.
Therefore, we will not need to add any HCl to adjust the pH in this case.
7. Assemble the final solution:
- Take a clean container and add 800 ml of water.
- Add 4.55 ml of glacial acetic acid (17.6 M) to the water.
- Add 12.7 ml of 1.000 M KOH to the water.
- Mix the solution thoroughly until all components are dissolved completely.
Now, you have prepared the 800 ml of the 0.100 M potassium acetate buffer with a pH of 4.46.