# What Is the pH of a Buffer System with H2PO4- and HPO42-?

• yolo123
In summary, the conversation discusses the creation of a buffer by combining 1g of H2PO4- and 1g of HPO42- in 100ml of H2O. The pH of the buffer is determined using Henderson's equation and the Ka/b values. The participants also discuss the role of H2PO4- and HPO42- as both an acid and a base in the system. The overall conclusion is that the buffer system is working as expected based on the given values. The handwriting of one participant is mentioned as being difficult to read, but the overall work is considered correct.
yolo123
Hello Forum!

1g of H2PO4- and 1 g of HPO42- are put together into 100 ml of H2O. What is the pH of the buffer created.

Ka1= 7.5x10^-3
Ka2=6.2x10^-8
Ka3=4.8x10^-13

______________________
Okay. I uploaded my solutions. (Please disregard the part 1.184 g/mol and the 34% on my answers sheet. That was part of another problem.) Basically, I decided to put down all the reactions and say that H2PO4- and HPO4- will be the important reactions according to Ka/b values.
Then, I used Henderson to get 7.21.
Are my steps of thinking that H2PO4- will not act as base because of Kb1, and that HPO4- will not act as acid because of Ka3 necessary and correct? Are they part of the reasoning that I should put on, say an exam?

Many thanks.

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Your handwriting is hard to read. And of course H2PO4- will be acting as a base in the system; the question is just to what extent. Same with HPO42-. This is both an acid and a base.

Yes, I looked at it from the point of view of the values of the Ka/b values. Does it seem to make sense? Sorry about my handwriting. This is actually my third "clean" version I made just for the forum.

Dear forum member,

Thank you for sharing your solutions. Your approach of using the Henderson-Hasselbalch equation to calculate the pH of the buffer is correct. The reasoning you provided for the choice of H2PO4- and HPO4- as the important reactions based on their Ka/b values is also valid.

Regarding your question about including the reasoning on an exam, it would depend on the specific instructions and requirements of the exam. If the question asks for a calculation of the pH using the Henderson-Hasselbalch equation, then providing the reasoning may not be necessary. However, if the question asks for an explanation of the choice of the buffer components, then including the reasoning would be important.

In general, it is always helpful to provide a brief explanation or reasoning for your calculations or choices in scientific problems, as it shows your understanding of the concepts and helps the reader follow your thought process. So including the reasoning in your answers would be a good practice.

I hope this helps. Keep up the good work in your studies!

## 1. What is the purpose of a buffer system?

A buffer system is designed to maintain a stable pH in a solution by resisting changes in acidity or alkalinity.

## 2. How does the H2PO4- HPO4- buffer system work?

The H2PO4- HPO4- buffer system works by utilizing the conjugate acid-base pair of dihydrogen phosphate (H2PO4-) and monohydrogen phosphate (HPO4-) to maintain a stable pH. When an acid is added to the solution, the H2PO4- will act as a weak base, consuming the excess H+ ions to prevent a significant change in pH. Similarly, when a base is added, the HPO4- will act as a weak acid, consuming the excess OH- ions to maintain a stable pH.

## 3. What is the optimal pH range for the H2PO4- HPO4- buffer system?

The optimal pH range for the H2PO4- HPO4- buffer system is between 6.2-7.2. This range is ideal because it falls within the physiological pH range of most biological systems.

## 4. Can the H2PO4- HPO4- buffer system be used in both acidic and basic solutions?

Yes, the H2PO4- HPO4- buffer system can be used in both acidic and basic solutions. However, its effectiveness may be limited in extremely acidic or basic solutions, as the buffer capacity decreases at extreme pH levels.

## 5. How can I prepare a H2PO4- HPO4- buffer solution?

To prepare a H2PO4- HPO4- buffer solution, you will need to mix equal amounts of a weak acid (such as sodium dihydrogen phosphate) and its conjugate base (such as disodium hydrogen phosphate) in water. The amount of acid and base used will depend on the desired pH of the solution and the desired buffer capacity.

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