Understanding pH after Mixing Hydrochloric Acid Solutions: A Practical Example

Hydrochloric acid (HCl) is one of the most common strong acids used in various industrial, laboratory, and educational settings. When mixing different concentrations and volumes of HCl solutions, it's crucial to understand the resulting pH. This article will explore the pH change in a specific scenario where 150 mL of an HCl solution is mixed with 100 mL of a 0.2 M HCl solution.

Introduction to pH and Hydrochloric Acid

HCl is a strong acid, and in its aqueous solution, it dissociates completely into hydrogen ions (H ) and chloride ions (Cl-). The pH of an HCl solution is a measure of the concentration of hydrogen ions in the solution and is defined as the negative logarithm of the hydrogen ion concentration:

pH -log[H ]

Molarity and Its Significance

Molarity (M) is a measure of the concentration of a solution and is defined as the number of moles of solute per liter of solution. In this case, we are dealing with a 0.2 M HCl solution. This means that 1 liter of the solution contains 0.2 moles of HCl.

Calculating Hydrogen Ion Concentration in the Solution

Since HCl is a strong acid, it dissociates completely, and the concentration of hydrogen ions in the solution is equal to the concentration of HCl.

[H ] 0.2 M

To find the pH, we use the formula:

pH -log(0.2) ≈ 0.70

Mixing Solutions and Its Effects on pH

When the 150 mL of an HCl solution is mixed with 100 mL of a 0.2 M HCl solution, the total volume of the solution becomes 250 mL. The key step is to calculate the total moles of HCl and then the new concentration.

Moles of HCl in 150 mL (0.150 L) * (Molarity of HCl) 0.150 * M

Moles of HCl in 100 mL (0.100 L) * (0.2 M) 0.02 M

Total moles of HCl 0.150 * M 0.02

New concentration of HCL in the 250 mL solution (0.150 * M 0.02) / 0.250 L

For the sake of clarity, let's assume the 150 mL solution is 1 M (a typical concentration for HCl solutions), though it can vary:

Total moles of HCl (0.150 * 1) 0.02 0.170 moles

New concentration of HCl 0.170 / 0.250 0.68 M

pH -log(0.68) ≈ 0.16

Conclusion

Mixing different volumes and concentrations of HCl solutions can significantly alter the pH of the final solution. Understanding these changes is crucial in various applications, from laboratory experiments to industrial processes. By calculating the moles and concentrations accurately, one can predict the resulting pH and ensure proper handling and disposal of the mixture.

Frequently Asked Questions

Q: What is the relationship between molarity and pH?

Molarity (M) is the measure of the concentration of a solution. pH is defined as the negative logarithm of hydrogen ion concentration. Therefore, higher molarity results in a lower pH, indicating a more acidic solution.

Q: Why is HCl considered a strong acid?

Hydrochloric acid is considered a strong acid because it dissociates completely in water, releasing all its hydrogen ions (H ) into the solution.

Q: How does the volume of solution affect the final pH?

The volume of solution affects the final concentration of ions in the solution. By diluting a concentrated solution, you can lower its pH or reduce its acidity.

References

1. House, W. H. (2020). Chemistry in Context 9th Edition. McGraw-Hill Education.

2. Oxtoby, D. W., Nachtrieb, N. H., comercial-boldspamcomercial-bold Perlman, H. P. (2019). Principles of Modern Chemistry 9th Edition. Cengage Learning.

3. Fraydounpour, F. (2018). Thorough Understanding of Chemistry Fundamentals. Wiley-Blackwell.