. Use the Henderson-Hasselbach equation:
pH = pKa + log [A-]/[AH], where A- is the conjugate base and AH is the weak acid.
Calculate the pH of an acetate buffer in which the acetate concentration [CH3COO-] is 3.5 times greater than acetic acid [CH3COOH]. pKa = 4.74.
9. How would you prepare a 0.6 M acetate buffer with a pH = 4.0?
pH = pKa + log(acetate)/(acetic acid)
You want (acetate)*3.5 = (acetic acid) so I would let CH3COOH = x and CH3COO^- = 3x
Substitute into the HH equation and solve for pH.
b.
Use the HH equation, substitute Pka and pH and solve for base/acid ratio.
The second equation you need is
acid + base = 0.6M
Solve those simultaneously to obtain base and acid, then use those values to prepare the solution from CH3COOH and CH3COONa (Note: You can use CH3COOH and NaOH but that requires more math.) Do it the easy way by weighing out CH3COOH and CH3COONa.
To calculate the pH of an acetate buffer using the Henderson-Hasselbach equation, we need to determine the respective concentrations of the acetate ion ([A-]) and the weak acid acetic acid ([AH]).
Given that the acetate concentration ([CH3COO-]) is 3.5 times greater than the acetic acid concentration ([CH3COOH]), we can express this relationship mathematically as:
[CH3COO-] = 3.5 * [CH3COOH]
To calculate the pH, we also need the pKa value of acetic acid, which is stated as 4.74.
Now, let's substitute the values into the Henderson-Hasselbach equation:
pH = pKa + log([A-]/[AH])
Since we want to find the pH, we'll rearrange the equation:
pH - pKa = log([A-]/[AH])
Now, substitute the values we know:
pH - 4.74 = log((3.5 * [CH3COOH])/[CH3COOH])
Next, simplify the equation:
pH - 4.74 = log(3.5)
To remove the logarithm, we can convert it to its antilogarithm:
10^(pH - 4.74) = 3.5
To solve for pH, isolate it by rearranging the equation:
pH = 4.74 + log(3.5)
Using a scientific calculator, calculate the logarithm of 3.5 and add it to 4.74 to find the pH of the acetate buffer.
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Now, let's address the second question regarding the preparation of a 0.6 M acetate buffer with a pH of 4.0.
To prepare a buffer solution, you need a weak acid and its conjugate base. In this case, the weak acid is acetic acid (CH3COOH), and its conjugate base is acetate (CH3COO-).
To make a 0.6 M acetate buffer, you need to determine the appropriate ratio of acetic acid to acetate that will yield a pH of 4.0.
Using the Henderson-Hasselbach equation:
pH = pKa + log([A-]/[AH])
We can rearrange for the desired ratio:
[A-]/[AH] = 10^(pH - pKa)
Now, substitute the known values:
[A-]/[AH] = 10^(4.0 - 4.74)
Using a scientific calculator, calculate the value of 10^(4.0 - 4.74), which is the ratio of [A-]/[AH].
Let's assume the calculated ratio is X.
Therefore, to prepare a 0.6 M acetate buffer, you need to mix 0.6 M acetic acid with X * 0.6 M acetate. This will give you the desired pH of 4.0.