# Buffer solution + equilibrium + Organic compound

• myvow
In summary, the question asks why the equilibrium constant (Ka) for the dissociation of acetic acid (CH3COOH) is calculated using the concentrations of acetate ions (CH3COO-) and hydrogen ions (H+) instead of using the initial concentrations modified by the extent of dissociation. This is because the initial concentrations are modified by the extent of dissociation, which is determined by the reaction between the acetate ions and hydrogen ions to form acetic acid. The presence of sodium acetate (CH3COONa) in the solution may have caused confusion, but it simply represents the composition of sodium acetate in aqueous solution and does not affect the calculations.
myvow
For this question,why Ka =(ch3coo-)(h+)/(ch3cooh) instead of Ka =(ch3coo +x)(x)/(ch3cooh-x) ?

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myvow said:
why Ka =(ch3coo-)(h+)/(ch3cooh) instead of Ka =(ch3coo +x)(x)/(ch3cooh-x)

First equation uses equilibrium concentrations, second uses initial concentrations modified by the extent of the dissociation, to calculate equilibrium concentrations. Do you understand what these means?

myvow said:
For this question,why Ka =(ch3coo-)(h+)/(ch3cooh) instead of Ka =(ch3coo +x)(x)/(ch3cooh-x) ?

You have now modified your initial post. I do not understand what you have in mind by your x.
You might have been confused by the CH3COONa. That is a conventional formula, it really just represents the composition of sodium acetate but in aqueous solution and for that matter in crystal sodium acetate is all Na+ and CH3COO-. When you add to this "HCl" - I.e. H+ + Cl- - a fraction of the acetate ions CH3COO- become protonated to form CH3COOH and then the calculations you are asked about involve that fraction, which actually determines the pH as you can see from the given formulae.

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The reason why the equilibrium constant, Ka, is expressed as Ka = (CH3COO-)(H+)/(CH3COOH) instead of Ka = (CH3COO+x)(x)/(CH3COOH-x) is because the buffer solution contains a significant amount of the weak acid, CH3COOH, and its conjugate base, CH3COO-. When a weak acid is in equilibrium with its conjugate base, the concentration of the acid and its conjugate base are constantly changing, but their ratio remains constant. This is known as the Henderson-Hasselbalch equation, which states that the ratio of the concentrations of a weak acid and its conjugate base is equal to the negative logarithm of the acid's dissociation constant (pKa). Therefore, the equilibrium constant, Ka, is expressed in terms of the initial concentrations of the acid and its conjugate base, as shown in the first equation. The second equation, on the other hand, assumes that there is a change in concentration of the acid and its conjugate base, which is not the case in a buffer solution. Therefore, the first equation is the correct representation of the equilibrium constant for a buffer solution.

## 1. 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 composed of a weak acid and its conjugate base (or vice versa) and is able to maintain a relatively constant pH by absorbing or releasing H+ ions.

## 2. How does a buffer solution maintain equilibrium?

A buffer solution maintains equilibrium by having a relatively large amount of both the weak acid and its conjugate base present. When an acid or base is added, the buffer components react with it to form a weaker acid or base, respectively. This shift in equilibrium helps to keep the overall pH of the solution stable.

## 3. Can buffer solutions be made using organic compounds?

Yes, buffer solutions can be made using organic compounds. Many organic acids, such as acetic acid, and their conjugate bases can be used to create buffer solutions. These types of buffers are commonly used in biological and biochemical experiments.

## 4. How do organic compounds affect the buffering capacity of a solution?

The buffering capacity of a solution can be affected by the type and concentration of organic compounds present. Generally, the higher the concentration of the buffer components, the greater the buffering capacity. Additionally, the pKa (acid dissociation constant) of the organic compound used can also impact the buffering capacity of the solution.

## 5. Can buffer solutions be used in non-aqueous systems?

Yes, buffer solutions can be used in non-aqueous systems. Non-aqueous buffer solutions are commonly used in organic chemistry experiments or in industrial processes. They are composed of a weak acid and its conjugate base in a non-aqueous solvent, such as an organic solvent or liquid paraffin.

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