# What Is the Approximate PH of a 0.10 M Solution of a Weak Acid That Has a Ka of 5 X 10-5 M?

What Is the Approximate pH of a 0.10 M Solution of a Weak Acid That Has a Ka of 5 x 10-5 M?

The pH of a solution can provide valuable information about its acidity or alkalinity. In the case of a weak acid, the pH is determined by its concentration and the dissociation constant, also known as Ka. In this article, we will explore the approximate pH of a 0.10 M solution of a weak acid with a Ka value of 5 x 10-5 M and provide answers to frequently asked questions regarding this topic.

Understanding Weak Acids and Ka:

Before delving into the calculation of pH, it is essential to understand the concept of weak acids and Ka. Weak acids are substances that only partially dissociate in water, meaning they do not fully give up their hydrogen ions (H+). In contrast, strong acids completely dissociate, resulting in a high concentration of H+ ions in the solution.

Ka, or the acid dissociation constant, is a measure of the extent to which a weak acid dissociates in water. It describes the equilibrium between the acid and its conjugate base. The higher the Ka value, the greater the extent of dissociation, indicating a stronger acid.

Calculating pH for a 0.10 M Solution of a Weak Acid:

To calculate the pH of a 0.10 M solution of a weak acid with a Ka value of 5 x 10-5 M, we need to consider the equilibrium between the weak acid and its conjugate base. The dissociation of the weak acid can be represented by the following equation:

HA ⇌ H+ + A-

The equilibrium expression for this reaction can be written as:

Ka = [H+][A-] / [HA]

Given that the initial concentration of the weak acid, [HA], is 0.10 M, and the concentration of [H+] is equal to the concentration of [A-] due to the 1:1 stoichiometry of the reaction, we can substitute these values into the equation:

5 x 10-5 = [H+]^2 / 0.10

Simplifying the equation, we find:

[H+]^2 = 5 x 10-6

Taking the square root of both sides, we get:

[H+] ≈ 7.07 x 10-3 M

Since pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration, we can calculate the pH:

pH ≈ -log(7.07 x 10-3) ≈ 2.15

Therefore, the approximate pH of a 0.10 M solution of a weak acid with a Ka value of 5 x 10-5 M is 2.15.

FAQs:

Q: What is the significance of pH in chemistry?

A: pH is a measure of the acidity or alkalinity of a solution. It is crucial for many chemical processes and reactions. pH affects the behavior of molecules, enzymes, and other biological systems.

Q: Is a weak acid more acidic than a strong acid?

A: No, a weak acid is less acidic than a strong acid. The acidity of an acid is determined by its ability to donate hydrogen ions. Strong acids fully dissociate, releasing a high concentration of hydrogen ions, making them more acidic than weak acids.

Q: How does the Ka value affect the pH of a weak acid solution?

A: The Ka value represents the extent of dissociation of a weak acid. A higher Ka value indicates a greater extent of dissociation, resulting in a higher concentration of H+ ions in the solution. Consequently, a higher Ka value leads to a lower pH, indicating a more acidic solution.

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Q: What are some examples of weak acids?

A: Examples of weak acids include acetic acid (CH3COOH), formic acid (HCOOH), citric acid (C6H8O7), and carbonic acid (H2CO3).

In conclusion, the approximate pH of a 0.10 M solution of a weak acid with a Ka value of 5 x 10-5 M is 2.15. Understanding the concept of weak acids, their dissociation constant, and how they affect pH is essential in various fields of chemistry and biology.