Calculate the weight of the buffers you will use to make the buffers for Part A for all the buffer possibilities listed under procedures in Part A, step 1. In other words, how many grams do you need to make 100 mL of a 0.1 M buffer?
Buffer pKa1 pKa2 pKa3 formula wt.(g/mol)
Acetate 4.76 136.1
CAPS 10.4 221.3
Citrate 3.06 4.74 5.40 294.1
HEPES 7.55 238.3
Phosphate 2.12 142.0
Tricine 8.15 179.2
TRIS 8.3 121.1
sorry, on phosphate pKa2 is 7.21 and pKa3 is 12.32
pH = pKa + log[(base)/(acid)]
plug in the pH you want and the pKa of the acid. The (base) is the concentration of the anion and concentraion of the acid is the acid (for acetate it is acetic acid).
For example acetic acid molar mass is 60. Therefore, 1 M solution of HAc (acetic acid) is 60 g/L. Thus a 0.1 M solution is 6 g/L or 0.60 g/100 mL.
Then pH = pKa + log (base/acid)
pH = 4.76 + log(base/0.1)
Plug in pH and calculate the acetate required. That will be in mols/L and you will need to change it to mols/100 mL, then to grams. Post your work if you get stuck.
which buffer do I choose? There are instructions above the table does this have anything to do with my choice for the problem i am working out, or do I just pick whatever one I want and do the math?
Above the table it states, Make two buffers starting with solid material, which is the most common way to make buffers. You will be given a desired pH, and your task is to prepare 100 ml of two appropriate buffers at a concentration of 0.10 M. One of the buffers will be a phosphate or citrate buffer, and the other will be one of the others (not phosphate or citrate).
Using the following table, choose the most appropriate buffer compounds for your pHs.
okay, this is what I have so far and now don't know how to finish.
I used TRIS.
pH=8.30+log(121.1/157.6)
pH=8.19
Note: TRIS (Trizma base) MW= 121.1
TRIS-HCL MW= 157.6 (I hope I am using the correct things)
After I got the pH, I got stuck on how to finish the problem.
The way I read the initial problem and your instructions--you were to do the calculation for ALL of the possibilities, then choose two of them. In your calculation above with TRIS, you CAN'T use molar masses for concentrations. molarity = mols/L
The problem also states that you will be given a desired pH. So plug in the pH and pKa values as well as the MOLARITY of the acid (or base), then calculate the MOLARITY of the other. Then you can convert the molarities to grams/100 mL.
these are pre-lab questions, so i haven't been given a ph yet. These are questions due before lab starts.
okay, let me just stick to the pre-lab question using the table. Here is the pre-lab question:
Calculate the weight of the buffers you will use to make the buffers for part A for all the buffer possibilities listed under procedures in part A, stp i. In other words, how many grams do you need to make 100 mL of a 0.1 M buffer?
How do I answer this question? I don't know anything about a pH.
Okay, I have worked out the first two. Am I doing them right?
For acetate, I got 27.9 grams. For CAPS, I got 5.08 grams.
Am I doing these right?
By the way, I have been using a pH of 7 for all of them.
I noticed some mistakes in my conversions. The answer I got for acetate was 37.97 grams. For CAPS, i got.112 grams. Are these correct? Thanks.
I don't think pH of 7 is the way to go. I know this is a pre-lab; however, it says you will be given a pH. Look at the procedures part A, Step 1, and see if they don't list a pH for some of those procedures. That will be the pH you want to use. The appropriate acid/base system you use will be the one with pKa closest to the pH from the procedures section.
Let me add that you will need a mass of BOTH the acid and the base. When you say you have some number for acetate, that doesn't tell me if that is the acetate salt (which salt by the way), or the acid, and the prof will want the OTHER one also (in grams/100 mL).
okay. Here is the first acid and base worked out.
Acetate(base): .3797 grams
Acetic acid(acid): .1674 grams
Are these right? If they are, i can finish the rest. Thanks.
Are you trying to get to 7.0?
Acetic acid/acetate is not a good system to use it as a buffer for pH = 7.0. But we can calculate it.
What did you use as the salt for the acetate? If I use sodium acetate (molar mass about 82) then 0.3797/82 = 0.00463 M and
0.1674 g acetic acid is 0.00279 M.
The pH = 4.76 + log (B/A) =
4.76 + 0.22 = 4.98
If 4.98 is where you are trying to go then your numbers are good EXCEPT that doesn't look like 0.1 M to me.