How do I find the volume of 6M stock solution of acetic acid needed to make a 250 mL buffer at a pH 4.7? pka is 4.74
What will you use for the base? Do you intend to add NaOH to acetic acid to make the salt or are you adding an amount of sodium acetate?
Adding 20mL sodium acetate, sorry.
And the concn of the sodium acetate is what?
And it's 0.20M
The Henderson-Hasselbalch equation is
pH = pKa + log[(base)/(acid)]
millimoles base = 20 mL x 0.2M = 4 mmoles.
4.7 = 4.74 + log (4/a)
(4/acid)= 0.91
acid = 4/0.91 = ??
M = mmoles/mL.
You know mmols and you know M, solve for mL and dilute to whatever volume you need. There is a caution that goes along with this. These are small amount in a relatively large volume; therefore, the buffering capacity may be limited. I ran a quick calculation on adding 10 mL of 0.1 M NaOH to the buffer and came out with about 4.35 which is not bad (the buffering capacity is defined as that amount of base or acid to change the pH by +/- 1.0 and 10 mL of 0.1M NaOH doesn't cross that line.)
Thank you so much!
To find the volume of a 6M stock solution of acetic acid needed to make a 250 mL buffer at a pH of 4.7, you need to use the Henderson-Hasselbalch equation. The Henderson-Hasselbalch equation relates the pH of a buffer solution to the pKa of its weak acid component and the ratio of the concentrations of the weak acid and its conjugate base.
Here are the steps to find the volume:
Step 1: Write down the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
Step 2: Rearrange the equation to solve for the ratio [A-]/[HA]:
[A-]/[HA] = 10^(pH - pKa)
Step 3: Plug in the given pH (4.7) and pKa (4.74) into the equation:
[A-]/[HA] = 10^(4.7 - 4.74)
Step 4: Calculate the value of [A-]/[HA]:
[A-]/[HA] = 10^(-0.04)
Step 5: Determine the ratio of the concentrations by simplifying the equation:
[A-]/[HA] = 0.9753
Step 6: Since a buffer solution is usually prepared by mixing equal volumes of weak acid and its conjugate base, the ratio of the concentrations can also be considered as the ratio of the volumes. In this case, the volume of the conjugate base ([A-]) is the volume of the 6M stock solution of acetic acid needed to make the buffer.
Step 7: Find the volume by multiplying the ratio by the total volume of the buffer solution:
Volume of acetic acid stock solution = (0.9753)(250 mL)
The calculated volume of the 6M stock solution of acetic acid needed to make the 250 mL buffer is approximately 243.8 mL.