Thank you all for stopping by again to learn some really cool chemistry with me. Today I'll be talking about the Henderson Hasselbalch Equation.
Why you should understand the Henderson Hasselbalch Equation:
Henderson Hasselbalch Equation is an essential piece of understanding buffers. Buffers making life possible, therefore Henderson Hasselbalch makes life possible.
Key Concepts
- Henderson Hasselbalch Equation
- How Henderson Hasselbalch is Useful
- Practice
Henderson Hasselbalch Equation
The Henderson Hasselbalch Equation is used to create a buffer solution. It takes into account the specifics of the conjugate acid/base pair present in solution as well as the desired pH of the buffer. In the lab, Henderson Hasselbalch is the way to calculate how to make a buffer.
To the left is the Henderson Hasselbalch equation with explanations of each part.
pH is what Henderson Hasselbalch Equation typically describes. The pH is of the resulting solution which contains the designated ratio of conjugate acid to base and the specific pKa of the weak acid.
How Henderson Hasselbalch is Useful
Making buffers is a common practice in the laboratory setting. Henderson Hasselbalch is the most common way scientists calculate how they make a buffer. There are typically two different ways this is done.
1. Calculate the ratio of conjugate base to weak acid given the desired pH of the buffer.
In the lab, the most likely situation to use Henderson Hasselbalch would be making a buffer from scratch. In this case, the desired pH of your buffer would be known. The desired pH typically depends on what system you're working in. Given the pH and the pKa of the weak acid being used, you can calculate the ratio of weak acid to conjugate base. This ratio would either be a whole number or a fraction. In the event of a whole number ratio, the whole number can be converted into a fraction by putting it over one. In the event of a decimal ratio, the decimal can then be converted to a fraction.
2. Calculate the resulting pH of the buffer from your concentrations of conjugate base and weak acid.
The second possible situation is that you could be given a ratio of conjugate base to acid and need to find the pH of the resulting solution.
Practice
Below I've attached a practice problem of how to make a buffer. The problem gives the following information: The desired pH of the buffer is 3.75 and the pKa of the weak acid is 4.72. What is the ratio of conjugate base to weak acid?
Step by Step Answer:
1. Using the Henderson Hasselbalch equation we can solve for the ratio of conjugate base to acid. In the practice problem above I've labeled the ratio with an x. This makes it easier to view the conjugate acid-base pair as a ratio rather than two individual concentrations.
2. First I plug in the values for pH and pKa. To solve for x, I need to get the logarithm with the x term alone. I can do this by subtracting the pKa term to the other side. This results in the 3.75 - 4.72 term.
3. Once the logarithm is alone on one side of the equation, raising each side to the "10" will get rid of it, leaving the x term alone. This results in 10^(-.97) which is 0.1072. Because the ratio of conjugate base to acid is less than one, there is more weak acid than there is conjugate base.
If the ratio of conjugate base to acid is a number less than one, there is more weak acid in the solution that there is base. If the ratio of conjugate base to acid is a number larger than one, there is more conjugate base than there is weak acid.
Summary
- The Henderson Hasselbalch Equation is used to create a buffer solution.
- Henderson Hasselbalch is the most common way scientists calculate how they make a buffer. There are typically two different ways this is done. Calculate the ratio of conjugate base to weak acid given the desired pH of the buffer. The second way is to calculate the resulting pH of the buffer from your concentrations of conjugate base and weak acid.
- If the ratio of conjugate base to acid is a number less than one, there is more weak acid in the solution that there is base. If the ratio is a number larger than one, there is more conjugate base than there is weak acid.
I hope you've enjoyed this post on the Henderson Hasselbalch Equation. If you have any questions feel free to comment on this post or message me. Additionally, you could comment on my Youtube video about Henderson Hasselbalch which I've attached below.
- Saren
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