Understanding pH and Temperature
* pH: A measure of the acidity or alkalinity of a solution. It's based on the concentration of hydrogen ions (H+).
* Temperature: As temperature increases, the dissociation of water molecules (H2O → H+ + OH-) increases. This means there are more hydrogen ions (H+) present, leading to a lower pH (more acidic).
Calculation
1. Ion Product of Water (Kw): At 25°C, Kw is 1.0 x 10^-14. However, Kw changes with temperature.
2. Kw at 70°C: You'll need a table or equation to find the Kw value at 70°C. The value is approximately 2.5 x 10^-13.
3. Calculate H+ Concentration:
* Kw = [H+][OH-]
* Since pure water has equal concentrations of H+ and OH-, we can simplify to: Kw = [H+]^2
* [H+] = √Kw = √(2.5 x 10^-13) ≈ 5.0 x 10^-7 M
4. pH Calculation:
* pH = -log[H+]
* pH = -log(5.0 x 10^-7) ≈ 6.3
Result: The pH of pure water at 70°C is approximately 6.3.
Important Note: Even though the pH is slightly lower than 7, pure water at 70°C is still considered neutral because the concentrations of H+ and OH- are equal.
