# Chemistry - Concentration from known pH and Ka.

• Mholnic-
In summary, the problem involves calculating the concentration of acetic acid in a sample of vinegar with a pH of 2.90, using the Ka value of 1.8 x 10-5 for acetic acid. By rearranging the equations and substituting the given values, the concentration of the acid is calculated to be 8.8 x 10-12. However, this is not the final answer and further steps are needed to arrive at the correct answer of 0.089 M.
Mholnic-

## Homework Statement

A particular sample of vinegar has a pH of 2.90. If acetic acid is the only acid that vinegar contains (Ka = 1.8 x 10-5), calculate the concentration of acetic acid in the vinegar.

## Homework Equations

Ka = [Products]/[Reactants]
pH = -log[H+]

## The Attempt at a Solution

I arranged the problem in my usual lazy way:
Acid + Water ---> Conjugate Base + Hydrogen Ions

Assuming that [H+] is equal to [Conjugate Base] I calculated the concentration of the conjugate base and hydrogen ions.

pH = -log[H+] = 2.90
[H+] = 10-2.90 = [Conjugate Base]

Then I plugged those numbers and the given Ka value into:

Ka = [products]/[reactants]

1.8 x 10-5 = [10-2.90]2/[Acid]

[10-2.90]2/1.8 x 10-5 = [Acid]

I get 8.8 x 10-12. The book says the answer is .089 M. Not quite sure where I'm going wrong.. maybe I am not using the correct equations or perhaps it's an overlooked error in algebra. Help!

Mholnic- said:
[10-2.90]2/1.8 x 10-5 = [Acid]

Equation is correct, but it doesn't yield 8.8x10-12.

Note that concentration of the acid that you can calculate in this step is not the final answer yet.

## What is concentration in chemistry?

Concentration in chemistry refers to the amount of a substance that is present in a given volume or mass of a solution. It can be measured in various units such as molarity, molality, or mass percent.

## How is concentration calculated from known pH and Ka?

The concentration of an acid or base can be calculated from known pH and Ka values using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the conjugate base and [HA] is the concentration of the acid.

## What is Ka in chemistry?

Ka, or the acid dissociation constant, is a measure of the strength of an acid in a solution. It is defined as the ratio of the concentration of the dissociated ions to the concentration of the undissociated acid.

## What is the relationship between pH and concentration?

The concentration of an acid or base in a solution can affect the pH of the solution. A higher concentration of an acid will result in a lower pH, while a higher concentration of a base will result in a higher pH. This is due to the fact that more acidic or basic molecules are present to donate or accept protons, respectively.

## Why is concentration important in chemistry?

Concentration is an important concept in chemistry because it helps us understand the properties and behavior of substances in a solution. It also allows us to accurately measure and control the amount of a substance present, which is crucial in many chemical reactions and processes.

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