Introduction to pH and Strong Acids

The pH scale is a measure of how acidic or basic a solution is, and it ranges from 0 to 14. A pH of 7 is neutral, while values below 7 indicate acidity and values above 7 indicate basicity. Strong acids completely dissociate in aqueous solutions, meaning that their concentration directly correlates to the pH level. Understanding how to calculate the pH of strong acids is essential in various fields, from laboratory work to industrial applications.

Example 1: Determining the pH of Hydrochloric Acid (HCl)

In a laboratory setting, you may need to prepare a solution of hydrochloric acid (HCl) for various experiments. Knowing the pH of this solution is crucial.

Given a 0.1 M solution of HCl, we can calculate the pH as follows:

• Step 1: Recognize that HCl is a strong acid that dissociates completely in water:

  HCl → H⁺ + Cl⁻

• Step 2: The concentration of hydrogen ions [H⁺] is equal to the concentration of the acid:

  [H⁺] = 0.1 M

• Step 3: Use the pH formula:

  pH = -log[H⁺] = -log(0.1) = 1

Thus, the pH of the 0.1 M HCl solution is 1.

Notes:

• If the concentration were increased to 0.5 M, the pH would be:

  pH = -log(0.5) ≈ 0.3

Example 2: Calculating pH of Sulfuric Acid (H₂SO₄)

Sulfuric acid is another commonly used strong acid in industrial applications, particularly in battery manufacturing. When calculating the pH of a 0.2 M sulfuric acid solution, it’s important to note that sulfuric acid is diprotic, meaning it can donate two protons.

• Step 1: For the first dissociation, H₂SO₄ → H⁺ + HSO₄⁻, we get:

  [H⁺] from this step = 0.2 M

• Step 2: For the second dissociation, HSO₄⁻ → H⁺ + SO₄²⁻, we assume complete dissociation for simplicity, which gives:

  [H⁺] from this step = 0.2 M

• Step 3: Total [H⁺] = 0.2 M + 0.2 M = 0.4 M

• Step 4: Calculate the pH:

  pH = -log(0.4) ≈ 0.4

Therefore, the pH of the 0.2 M sulfuric acid solution is approximately 0.4.

Notes:

• If you were to dilute the solution to 0.05 M, the resulting pH would change accordingly:

  pH = -log(0.1) = 1

Example 3: pH Calculation for Nitric Acid (HNO₃)

Nitric acid is a strong acid widely used in fertilizers and explosives. Let’s consider a 0.25 M nitric acid solution.

• Step 1: Since HNO₃ is a strong acid, it also completely dissociates:

  HNO₃ → H⁺ + NO₃⁻

• Step 2: The concentration of hydrogen ions [H⁺] is thus:

  [H⁺] = 0.25 M

• Step 3: Calculate the pH:

  pH = -log(0.25) ≈ 0.60

Consequently, the pH of a 0.25 M nitric acid solution is approximately 0.60.

Notes:

• Higher concentrations will yield lower pH values, highlighting the direct relationship between acid strength and pH.

By understanding these practical examples of calculating pH of strong acids, one can better appreciate the behavior of acids in various chemical and real-world contexts.