# Calculate pH of a buffer solution

• Krushnaraj Pandya
In summary, to calculate the pH of a solution containing 0.1 mole of Ch3cooh, 0.2 mol of CH3COONa and 0.05 mol of naoh in 1 L, use the Henderson - Hasselbalch equation with the pKa for Ch3cooh (4.74). The resulting pH is 5.44, which is reasonably accurate even though there is a slight difference due to the assumption of neutralization going to completion. As a rule of thumb, if the acid has a pKa above 3 and is not too diluted, there is no need to worry about this difference. If in doubt, calculated values can be used as initial concentrations
Krushnaraj Pandya
Gold Member

## Homework Statement

Calculate pH of a solution containing 0.1 mole of Ch3cooh, 0.2 mol of CH3COONa and 0.05 mol of naoh in 1 L. (Pka for Ch3cooh=4.74).
2. The attempt at a solution
The 0.05 mol of NaOH will react with the 0.10 mol CH3COOH to produce 0.05 mol CH3COONa , and there will bve 0.05 mol CH3COOH remaining unreacted . The solution then contains:
0.05 mol CH3COOH
0.25 mol CH3COONa.
dissolved in 1.0L solution - these figures are the molarity of the compounds.
Use Henderson - Hasselbalch equation:
pH = pKa + log ([salt]/[acid])
pH = 4.74 + log (0.25 / 0.05)
pH = 4.74 + log 58.0
pH = 4.74+ 0.70
pH = 5.44

I'm getting the correct answer this way but my question is that since some moles of ch3cooNa are already present in the beginning, won't that hinder more formation of the same salt and therefore the concentration of the salt calculated is actually more than the actual value?

If you do the exact equilibrium calculation you will find that yes, there is a slight difference and the equilibrium is a bit shifted. But typically it doesn't matter and the pH calculated based on the assumption neutralization goes to completion is reasonably accurate.

See the examples here: http://www.chembuddy.com/?left=buffers&right=composition-calculation

Borek said:
If you do the exact equilibrium calculation you will find that yes, there is a slight difference and the equilibrium is a bit shifted. But typically it doesn't matter and the pH calculated based on the assumption neutralization goes to completion is reasonably accurate.

See the examples here: http://www.chembuddy.com/?left=buffers&right=composition-calculation
how do I decide where I can assume full dissociation in spite of other factors and where I cannot?

As a rule of thumb if the acid has pKa above 3 and is not too diluted (say, above 10-3 M) there is no need to worry.

If in doubt you can always use calculated values as initial concentrations for ICE table and see what that produces (http://www.chembuddy.com/?left=buffers&right=with-ICE-table).

Thank you

## What is a buffer solution?

A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added. It is made by combining a weak acid and its conjugate base, or a weak base and its conjugate acid, in equal amounts.

## How do you calculate the pH of a buffer solution?

To calculate the pH of a buffer solution, you need to know the concentrations of the weak acid and its conjugate base, or the weak base and its conjugate acid, as well as the pKa (acid dissociation constant) of the weak acid or base. You can use the Henderson-Hasselbalch equation: pH = pKa + log([conjugate base]/[weak acid]).

## What is the importance of a buffer solution in chemistry?

Buffer solutions are important in chemistry because they help to maintain a stable pH, which is necessary for many chemical reactions. They are commonly used in biological systems, such as in the human body, to regulate pH and prevent drastic changes that could be harmful.

## What factors can affect the pH of a buffer solution?

The pH of a buffer solution can be affected by changes in the concentration of the weak acid or base, the addition of a strong acid or base, or changes in temperature. It is important to maintain the concentrations and temperature of a buffer solution to ensure its effectiveness in maintaining a stable pH.

## Can a buffer solution have a pH of 7?

Yes, a buffer solution can have a pH of 7 if the concentrations of the weak acid and its conjugate base, or the weak base and its conjugate acid, are equal and their pKa values are also equal. This means that the buffer solution is at its optimal buffering capacity and can resist changes in pH effectively.

• Biology and Chemistry Homework Help
Replies
3
Views
2K
• Biology and Chemistry Homework Help
Replies
20
Views
2K
• Biology and Chemistry Homework Help
Replies
2
Views
1K
• Biology and Chemistry Homework Help
Replies
7
Views
3K
• Biology and Chemistry Homework Help
Replies
23
Views
2K
• Biology and Chemistry Homework Help
Replies
16
Views
2K
• Biology and Chemistry Homework Help
Replies
7
Views
3K
• Biology and Chemistry Homework Help
Replies
11
Views
2K
• Biology and Chemistry Homework Help
Replies
6
Views
3K
• Biology and Chemistry Homework Help
Replies
15
Views
3K