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AP Chemistry · Question of the Day

AP Chemistry Question of the Day

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Friday, August 7, 2026

A 6.0×10−4 M6.0\times10^{-4}\,\text{M} solution of Sr(OH)2(aq)_2(aq) is prepared at 25∘C25^\circ\text{C}. Assuming complete dissociation, what is [OH−][\text{OH}^-] in the solution?​

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A 6.0×10−4 M6.0\times10^{-4}\,\text{M} solution of Sr(OH)2(aq)_2(aq) is prepared at 25∘C25^\circ\text{C}. Assuming complete dissociation, what is [OH−][\text{OH}^-] in the solution?​

  1. 6.0×10^-8
  2. 6.0×10^-4
  3. 1.0×10^-14
  4. 1.2×10^-3 (correct answer)
  5. 3.0×10^-4

Explanation: This question tests pH and pOH of strong acids and bases. Sr(OH)₂ is a strong base that completely dissociates as Sr(OH)₂ → Sr²⁺ + 2OH⁻, producing 2 moles of OH⁻ per mole of Sr(OH)₂. With [Sr(OH)₂] = 6.0×10⁻⁴ M, we calculate [OH⁻] = 2 × 6.0×10⁻⁴ = 1.2×10⁻³ M. A common mistake is forgetting the stoichiometric coefficient and using [OH⁻] = 6.0×10⁻⁴ M (choice A). For bases containing multiple hydroxide ions, multiply the base concentration by the number of OH⁻ ions in the formula.