Given the following hydronium ion concentrations at 25°C, calculate the hydroxide ion concentration and classify each solution as acidic or basic: a. [H₃O⁺] = 1.5 × 10⁻³ M, b. [H₃O⁺] = 3.5 × 10⁻⁹ M, c. [H₃O⁺] = 8.3 × 10⁻¹¹ M. Which of the following correctly classifies solution b?

• A. [OH⁻] = 2.9 × 10⁻⁵ M, basic
• B. [OH⁻] = 2.9 × 10⁻⁵ M, acidic
• C. [OH⁻] = 2.9 × 10⁻⁶ M, basic
• D. [OH⁻] = 2.9 × 10⁻⁶ M, acidic

Answer: (C)

Verified step by step guidance

1. To find the hydroxide ion concentration [OH⁻] from the hydronium ion concentration [H₃O⁺], use the water dissociation constant at 25°C: K_w = 1.0 × 10⁻¹⁴. The relationship is given by the equation: [H₃O⁺] × [OH⁻] = K_w.
2. Rearrange the equation to solve for [OH⁻]: [OH⁻] = K_w / [H₃O⁺].
3. For each given [H₃O⁺], substitute the value into the equation to calculate [OH⁻]. For example, for part a, substitute [H₃O⁺] = 1.5 × 10⁻³ M into the equation.
4. After calculating [OH⁻] for each solution, compare the concentrations of [H₃O⁺] and [OH⁻] to determine if the solution is acidic or basic. A solution is acidic if [H₃O⁺] > [OH⁻] and basic if [OH⁻] > [H₃O⁺].
5. For solution b, calculate [OH⁻] using [H₃O⁺] = 3.5 × 10⁻⁹ M. Compare the calculated [OH⁻] with the given options to classify the solution correctly. The correct classification is when [OH⁻] = 2.9 × 10⁻⁶ M, indicating the solution is basic.