Understanding the Concepts
* Kw (Ionic Product of Water): Kw is the equilibrium constant for the auto-ionization of water:
```
2H₂O(l) ⇌ H₃O⁺(aq) + OH⁻(aq)
```
Kw = [H₃O⁺][OH⁻]
* pH: pH is a measure of the acidity or alkalinity of a solution. It is defined as:
```
pH = -log[H₃O⁺]
```
Calculations
1. Find [H₃O⁺]: Since pure water is neutral, [H₃O⁺] = [OH⁻]. We can use Kw to solve for [H₃O⁺]:
```
Kw = [H₃O⁺][OH⁻] = [H₃O⁺]²
[H₃O⁺] = √Kw = √(2.917 x 10⁻¹⁴) = 1.708 x 10⁻⁷ M
```
2. Calculate pH:
```
pH = -log[H₃O⁺] = -log(1.708 x 10⁻⁷) ≈ 6.77
```
Therefore, the pH of pure water at 313 K is approximately 6.77.
Important Note: Even though the pH is slightly less than 7, pure water at this temperature is still considered neutral. The pH of pure water is only exactly 7 at 298 K (25°C).
