Points to Remember:
- pH scale and its logarithmic nature.
- Strong acid dissociation.
- Calculation of pH using the formula: pH = -logââ[Hâº].
Introduction:
The question requires a factual and analytical approach to calculate the change in pH of pure water upon addition of a strong acid like HCl. Pure water has a neutral pH of 7 at 25°C, meaning the concentration of hydrogen ions ([Hâº]) and hydroxide ions ([OHâ»]) are both 1 x 10â»â· M. Adding an acid increases the [Hâº], thereby lowering the pH. We will use the principles of strong acid dissociation and pH calculation to determine the change.
Body:
1. Strong Acid Dissociation:
Hydrochloric acid (HCl) is a strong acid, meaning it completely dissociates in water into its constituent ions:
HCl(aq) â Hâº(aq) + Clâ»(aq)
This means that a 0.001 M HCl solution will have a hydrogen ion concentration of approximately 0.001 M (ignoring the contribution of H⺠from water autoionization, which is negligible in this case).
2. pH Calculation:
The pH of a solution is calculated using the formula:
pH = -logââ[Hâº]
Before adding HCl, the [Hâº] in pure water is 1 x 10â»â· M. Therefore, the initial pH is:
pH (initial) = -logââ(1 x 10â»â·) = 7
After adding 0.001 M HCl, the [Hâº] becomes approximately 0.001 M (1 x 10â»Â³ M). The new pH is:
pH (final) = -logââ(1 x 10â»Â³) = 3
3. Change in pH:
The change in pH is the difference between the final and initial pH:
ÎpH = pH (final) – pH (initial) = 3 – 7 = -4
Therefore, the pH of the pure water decreases by 4 units.
Conclusion:
Adding 0.001 M HCl to pure water significantly lowers the pH from 7 to 3, representing a decrease of 4 pH units. This demonstrates the impact of even small concentrations of strong acids on the acidity of a solution. This calculation assumes complete dissociation of HCl and ignores the minute contribution of water’s autoionization to the [Hâº]. In real-world scenarios, slight deviations might occur due to factors like temperature and ionic strength. Understanding pH changes is crucial in various fields, including environmental monitoring, chemical engineering, and biological systems, highlighting the importance of accurate calculations and careful consideration of relevant factors. Maintaining a balanced approach to environmental protection and responsible chemical handling is essential for sustainable development.
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