Algebra of the Henderson-Hasselbach equation

[helpme12345]

I am weak on the algebra of the Henderson-Hasselbach equation.

I have a prelab question:

Using the H-H equation, calculate the volumes of 0.1 MM phosphate solutions to mix to prepare 100ml each of buffers with pH values 6.0, 6.5, 7.0,7.5, and 8.0 starting with 01.M stock solutions of NaH2HPO4 and Na2HPO. The relevant reaction is given below:

H2PO4- -->HPO4^2- + H+

The pKa is 7.21.

I know the pH is the values listed.

pH=pKa+log{[A-]/[HA]}

How do I start this problem?

 

[Mike H]

You know the pKa, and you know what the pH needs to be for each of the buffers. What you need to know is the ratio of the phosphate stock solutions, so you can then mix up the appropriate volumes. So, you get your known values on one side and your unknown values on another side. Once you have that, you need to get rid of the logarithm (base 10) using its inverse. Once you've done that, then it's just a matter of calculating what the ratio is equal to (which you should be able to do on a calculator, it'll be a little messy) and then figuring out the appropriate volumes, dilutions, and the rest.

I hope this helps without giving too much away...

 

[ravenprp]

The Henderson-Hasselbach equation is an important tool in biochemistry and is used to calculate the pH of a buffer solution based on the concentration of the acid and its conjugate base. In this case, we are given a specific scenario where we need to calculate the volumes of two phosphate solutions to prepare buffers with different pH values. Let's break down the problem step by step:

1. Write out the Henderson-Hasselbach equation: pH = pKa + log([A-]/[HA])

2. Identify the relevant acid and conjugate base in the reaction given. In this case, it is H2PO4- and HPO4^2-.

3. Determine the pKa value. In this case, it is given as 7.21.

4. Set up the equation for each desired pH value. For example, for a pH of 6.0, the equation would be: 6.0 = 7.21 + log([HPO4^2-]/[H2PO4-])

5. Rearrange the equation to solve for the ratio [HPO4^2-]/[H2PO4-]. In the example of pH 6.0, the equation would become: [HPO4^2-]/[H2PO4-] = 10^(6.0-7.21)

6. Calculate the ratio for each desired pH value.

7. Use the ratio to determine the volumes of the two phosphate solutions needed to prepare 100ml of each buffer. Remember, the total concentration of the buffer solution will be the sum of the concentrations of the acid and conjugate base.

I hope this helps you understand the algebraic steps involved in solving this problem. Remember to always double check your calculations and units to ensure accuracy. Good luck!