Calculation and Understanding of pH in Water with Added Nitric Acid

In a laboratory setting, modifying the pH of water through the addition of various substances can be a valuable educational tool. This article explores the pH calculation in a solution where nitric acid (HNO3) is added to distilled water. Understanding the principles behind these calculations is crucial for chemists and students studying chemistry.

Introduction to pH and Acids

The pH of a solution is a measure of its acidity or basicity on a scale from 0 to 14. Pure water at 25°C has a neutral pH of 7.00, which means it is neither acidic nor basic. Nitric acid (HNO3) is a strong acid that dissociates completely in water, releasing hydronium ions (H3O ) and nitrate ions (NO3-).

Scenario 1: Adding No HNO3 to Distilled Water

Consider a scenario where you add zero moles of HNO3 to 1 liter of pure distilled water. Under these conditions, the pH of the solution remains at 7.00. This is because no additional H ions are introduced into the solution, and the natural pH of distilled water is 7.00.

Mathematical Representation:

[ text{pH} -log[H^ ] ]

In 1 liter of pure water, the concentration of H ions is (1.0 times 10^{-7} , text{M}) at 25°C, resulting in a pH of 7.00.

Scenario 2: Adding a Small Quantity of HNO3

Now, let's consider a scenario where a small quantity of HNO3 is added to the water. The exact quantity of HNO3 is crucial for determining the pH of the solution. Nitric acid dissociates completely in water, according to the equation:

[ text{HNO}_3 rightarrow text{H}^ text{NO}_3^- ]

The dissociation of HNO3 increases the concentration of H ions in the solution, thereby lowering the pH.

Calculating pH

Let's assume you add 1.0 x 10-8 moles of HNO3 to 1 liter of water. The concentration of H ions will be equal to the moles of HNO3 added, as the dissociation is complete:

[ [H^ ] 1.0 times 10^{-8} , text{M} ]

Using the pH formula, we can calculate the pH of the solution:

[ text{pH} -log(1.0 times 10^{-8}) 8 ]

Therefore, the pH of the solution will be 8, indicating a slightly basic solution due to the low concentration of H ions.

Practical Implications

Understanding the pH calculation in solutions with added acids is essential in various applications, such as environmental monitoring, water quality testing, and industrial processes. Accurate pH measurements help in maintaining the correct conditions for various chemical reactions and biological systems.

Conclusion

In summary, the pH of pure distilled water is 7.00. When a small quantity of HNO3 is added, the concentration of H ions increases, resulting in a pH of 8. This basic understanding is fundamental for chemists and scientists, ensuring that they can accurately measure and manipulate pH levels in their experiments.

References

1. Zumdahl, S. S., Zumdahl, S. A. (2009). Chemistry. 7th edition. Brooks/Cole, Cengage Learning.

2. Brady, J. J., Carniello, T. E. (2013). Chemistry: The Central Science. 12th edition. Pearson Education.