The pKa of HEPES is 7.55 at 20C and it's MW is 238.31. Calculate the amounts of HEPES in grams and of 1.0M NaOH in ml that would be needed to make 300ml of 0.2M HEPES buffer at 7.2.
I'm not sure how to go about doing this problem.
Help please.
See if this will help you.
http://www.science.smith.edu/departments/Biochem/Biochem_353/Buffer_Prep.html#top
There is a better site but I can't find it now. Its done by Haverford College (UK).
I'll see how I do with that site, first.
Thanks Dr.Bob
I found the other site. Here it is.
http://www.haverford.edu/chem/Scarrow/GenChem/acidbase/buffer_prep.html
The usual biochemistry buffers means one thing and analytical chemists express buffers another way (or I may be out of touch with the new day thinking). At any rate, a 0.1 M buffer in the biochemistry sense means that base + acid = 0.1; i.e., 0.1 is the total of the species in whatever form they appear, such as acid and conjugate base. My experience in analytical chemistry is for 0.1 M acetic acid buffer to mean 0.1 M in acetic acid and we calculate how much acetate to add to make it the desired pH(or how much NaOH to add to make the acetate). You will need to decide which of these interpretations to use when solving the problem.
To calculate the amounts of HEPES and NaOH needed to make a 0.2M HEPES buffer at pH 7.2, we need to follow a few steps:
1. Determine the moles of HEPES needed:
To find the moles of HEPES, we can use the formula:
moles = Molarity × Volume
moles = 0.2 M × 300 ml
moles = 60 ml
2. Convert moles of HEPES to grams:
We need to convert the moles of HEPES to grams using its molecular weight (MW = 238.31 g/mol):
grams = moles × MW
grams = 60 ml × 238.31 g/mol
grams = 14,298.6 g
3. Calculate the volume of 1.0M NaOH needed:
Since the pKa of HEPES is given, we can use it to calculate the amount of NaOH needed to adjust the pH to 7.2. The pKa of HEPES (7.55) is close to the desired pH (7.2). Therefore, we need to neutralize the buffer by adding an equal amount of NaOH to bring the pH down.
To calculate the volume of 1.0M NaOH needed, we use the formula:
Volume of NaOH (ml) = Moles of HEPES × MW of HEPES / Molarity of NaOH
Volume of NaOH (ml) = 60 ml × 238.31 g/mol / 1.0 M
Volume of NaOH (ml) = 14,298.6 ml
So, to make 300 ml of 0.2M HEPES buffer at pH 7.2, you will need approximately 14.3 grams of HEPES and 14.3 ml of 1.0M NaOH. Keep in mind that this calculation assumes the HEPES solution is prepared in water. If you are using a different solvent or there are other factors involved, please adjust accordingly.