One liter of buffered solution should be prepared with a general concentration of 0.320M with a pH of 5.83 :
- Pyridine solution (formula: C5H5N) in water at a concentration of 1.00M - HCl solution in water at a concentration of 0.85M - Distilled water In addition, given: pKb (pyridine) = 8.75 What are the volumes to be taken from each ingredient to prepare the buffer?
You need two equations to start.
Here is equation 1 where Py is pyridine and HPy is when HCl is added. Remember that pKa + pKb = pKw = 14 so pKa is 14-8.75 = 5.25
pH = pKa + log (py/Hpy)
5.83 = 5.25 + log base/acid
Solve for B/A.
Equation 2 is
A + B = 0.320
Solve those two equations simultaneously for A and B (acid and base or HPy and Py)
So you will start with 320 millimoles Py (obviouly that is 320 mL of 1 M Py) and add x mL of 0.85 M HCl to get the final acid concentration you want.
I assume you can get the rest of this on your own. Post your work if you get stuck. BTW, you won't have a liter of solution when you finish the alculations so you will need to add water to make it up to the desired 1 L.
thank you very much for your help
To prepare the buffer solution, we need to determine the volumes of pyridine solution, HCl solution, and distilled water to be taken.
First, let's calculate the volume of pyridine solution needed. We know the desired concentration of the buffer (0.320M) and we have pyridine solution with a concentration of 1.00M. We can use the formula:
C1V1 = C2V2
where C1 is the initial concentration (1.00M), V1 is the volume to be taken from the pyridine solution (unknown), C2 is the desired concentration (0.320M), and V2 is the final volume (1 liter or 1000 mL).
Plugging in the values, we have:
(1.00M)(V1) = (0.320M)(1000 mL)
Solving for V1:
V1 = (0.320M)(1000 mL) / (1.00M)
V1 = 320 mL
So, we need to take 320 mL of pyridine solution.
Next, let's calculate the volume of HCl solution needed. We know its concentration (0.85M) and we want the final buffer to have a pH of 5.83. To do this, we need to calculate the concentration of the conjugate acid of pyridine (pyridinium ion) using the pKb value.
The equation to calculate the concentration of the conjugate acid is:
[Pyridinium] = pKb / (10^(pH - pKa))
Here, pKa is the negative logarithm of the acid dissociation constant of pyridine. Since the value of pKa is not provided, we can assume that it's the same as pKb. So, pKa = pKb = 8.75.
Substituting the values, we have:
[Pyridinium] = 8.75 / (10^(5.83 - 8.75))
[Pyridinium] = 8.75 / (10^(-2.92))
[Pyridinium] = 8.75 / 0.001271
[Pyridinium] ≈ 6885.20 M
To maintain a 0.320M concentration in the final buffer, we need to subtract [Pyridinium] from the desired concentration:
[HCl] = 0.320M - 6885.20 M
[HCl] ≈ -6885.00 M (negative concentration means it's actually the desired concentration in the opposite direction)
Now, we can calculate the volume of HCl solution needed:
C1V1 = C2V2
(-6885.00 M)(V1) = (0.85 M)(1000 mL)
Solving for V1:
V1 = (0.85 M)(1000 mL) / (-6885.00 M)
V1 ≈ -124 mL (again, negative volume means it's the desired volume in the opposite direction)
Since we can't measure a negative volume, we'll consider it as 0 mL. This means we don't need to add any HCl solution to the buffer.
Finally, to determine the volume of distilled water needed, we subtract the volumes of pyridine solution and HCl solution from the total volume (1 liter or 1000 mL):
Volume of distilled water = 1000 mL - 320 mL - 0 mL
Volume of distilled water = 680 mL
So, to prepare the buffer, you need to take 320 mL of pyridine solution and add 680 mL of distilled water.