Sorry!
HEPES, is a commonly used component of physiological buffers. The PKa of the protonated amino group of HEPES is 7.55 at 25C.
Describe the preparation of 1.00L of 0.100M HEPES buffer, pH 8.00, from the sodium salt of the basic form of HEPES (MW 260.3), 6.0M HCl and deionized water
B = base
A = acid = BH^+
B.Na + H^+ ==> A (BH^+)
pH = pKa + log[(base)/(acid)]
Plug in 8.00 for pH, 7.55 for pKa, and solve for ratio B/A.
You need another equation since there are two unknowns; and that one is A + B = 0.1M
Solve for A and B.
Then I would start with 0.1 mole B.Na which is 0.1 x 260.3 = ?? grams (I think about 26 grams). You know the concn needed for the acid and that MUST equal the H^+ needed. That will give you L of the 6M HCl needed and that should do it. Post your work if you get stuck. I arrived at about 26 grams of the sodium salt and about 4.5 mL of the acid (actually a little less), then make the solution to a final volume of 1.00 L in a volumetric flask.
I am a little confused on the part after you solve for 26 grams. Can you explain a little more? Thanks
To prepare a 0.100 M HEPES buffer solution with a pH of 8.00, you will need to calculate the amounts of HEPES sodium salt (cited as a source of the basic form of HEPES), 6.0 M HCl, and deionized water required.
Here's the step-by-step process to prepare the buffer solution:
Step 1: Determine the amount of HEPES sodium salt needed.
The molecular weight (MW) of HEPES is given as 260.3 g/mol. Since the pH of the buffer needs to be 8.00, we want the basic form of HEPES to be in excess. To achieve this, we need to calculate the amount of HEPES sodium salt required to achieve a final concentration of 0.100 M HEPES.
The acid dissociation reaction of HEPES in water can be represented as follows:
HEPES ⇌ H+ + HEPES-
The pKa value of the protonated amino group of HEPES is given as 7.55 at 25°C. To achieve a pH of 8.00, we need to consider the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
Since we want [A-]/[HA] to be at least 10,000 (to ensure that the HEPES base is in excess), we can calculate the necessary amount of HEPES sodium salt using the equation:
[A-]/[HA] = 10^3.45 (pH - pKa)
[A-]/[HA] = 10^3.45 (8.00 - 7.55)
[A-]/[HA] = 30,204
Therefore, the concentration of the basic form (HEPES-) is approximately 0.100 M / 30,204 = 3.31 x 10^-6 M.
To prepare 1.00 L of 0.100 M HEPES buffer, the amount of HEPES sodium salt required can be calculated using the formula:
m (mass) = M (molar mass) × V (volume) × C (concentration)
m (mass) = 260.3 g/mol × 1.00 L × 3.31 x 10^-6 M
m ≈ 8.61 mg
Therefore, measure approximately 8.61 mg of HEPES sodium salt (MW = 260.3 g/mol) using a balance.
Step 2: Prepare the required amount of 6.0 M HCl.
Since the HEPES sodium salt will be protonated by HCl to form the HEPES acid (HA), we need to determine the volume of 6.0 M HCl required to achieve a pH of 8.00.
Using the Henderson-Hasselbalch equation, we can rewrite it as:
[A-]/[HA] = 1/[10^3.45 (pKa - pH)]
[A-]/[HA] = 1/[10^3.45 (7.55 - 8.00)]
[A-]/[HA] = 1/316.227
Therefore, the concentration of the acidic form (HA) is approximately 0.100 M / 316.227 = 3.16 x 10^-4 M.
To neutralize the basic form of HEPES, we need to equate the moles of HEPES and HCl:
moles of HEPES sodium salt = moles of HCl
0.100 M × 1.00 L = 3.16 x 10^-4 M × V (volume of HCl)
V ≈ 317 L
Therefore, measure approximately 317 mL of 6.0 M HCl using a graduated cylinder or volumetric flask. Be cautious when handling concentrated acids and follow safety guidelines.
Step 3: Prepare the buffer solution.
Take a clean container (preferably a volumetric flask) and add the previously measured quantities of HEPES sodium salt (approximately 8.61 mg) and 6.0 M HCl (approximately 317 mL) to it. Then, add deionized water gradually while stirring until the total volume reaches 1.00 L. Ensure that all solid HEPES is dissolved completely.
Once the solution is prepared, you will have a 0.100 M HEPES buffer with a pH of 8.00.
Remember to always double-check your calculations and measurements to ensure accuracy in the preparation of laboratory solutions.