Millimoles of calcium hydroxide required to produce 100 mL of the aqueous solution of pH 12 is x×10⁻¹. The value of x is ___________ (Nearest integer).
Assume complete dissociation.

Solution

Chemistry ma’am · 4 steps

Go from pH to hydroxide, then remember each calcium hydroxide gives two hydroxide ions.

1. Find the hydroxide. At 25 °C, pH + pOH = 14, so pH 12 means pOH 2. That gives [OH⁻] = 10⁻² M.
2. Millimoles of hydroxide. Millimoles = molarity × volume in mL = 10⁻² × 100 = 1 millimole of OH⁻.

   Ca(OH)₂ → Ca²⁺ + 2OH⁻

3. Two hydroxides each. Each Ca(OH)₂ releases two OH⁻ ions, so you need half as much base: 1/2 = 0.5 millimole = 5 × 10⁻¹ millimole.
4. So. x = 5.

The common slip: 10. Forgetting that one Ca(OH)₂ gives two OH⁻ leaves the answer at 1 millimole, which is 10 × 10⁻¹ and gives 10, not 5.

The idea to keep. For a base, pH gives you [OH⁻] through pOH, not the base concentration itself. Divide by the number of OH⁻ each formula unit releases.

Answer from the NTA final answer key. Solution by PrepWiser.