Key Concepts
* Neutral Solution: Pure water is neutral, meaning it has equal concentrations of hydronium (H₃O⁺) and hydroxide (OH⁻) ions.
* Kw (Ion Product Constant): At 25°C, the product of the hydronium and hydroxide ion concentrations is a constant (Kw) equal to 1.0 x 10⁻¹⁴.
The Correct Expressions
1. [H₃O⁺] = [OH⁻] = 1.0 x 10⁻⁷ M: This is the most common way to express the concentrations. Since Kw = [H₃O⁺][OH⁻], and Kw = 1.0 x 10⁻¹⁴, both concentrations are equal to the square root of Kw, which is 1.0 x 10⁻⁷ M.
2. pOH = pH = 7: The pH and pOH are calculated as follows:
* pH = -log[H₃O⁺]
* pOH = -log[OH⁻]
* Since [H₃O⁺] = [OH⁻], pH = pOH = 7.
The Incorrect Expression
The expression that's incorrect is the one that contradicts the above principles.
Let's see some examples:
* [H₃O⁺] = 1.0 x 10⁻⁵ M and [OH⁻] = 1.0 x 10⁻⁹ M: This is incorrect because the product of these concentrations is 1.0 x 10⁻¹⁴, which is consistent with Kw, but the concentrations are not equal.
In summary:
The correct expressions for hydroxide and hydronium ion concentrations in pure water at 25°C emphasize the equal concentrations and the constant value of Kw. The incorrect expression would be one that doesn't reflect these fundamental principles.
