The neutralization occurs when 10 mL of 0.1M acid 'A' is allowed to react with 30 mL of 0.05 M base M(OH)₂. The basicity of the acid 'A' is __________.
[M is a metal]

Solution

Chemistry ma’am · 4 steps

At neutralization, the H⁺ given by the acid equals the OH⁻ given by the base, so count both in millimoles.

1. Neutralization condition. The acid and base neutralize exactly when the millimoles of H⁺ equal the millimoles of OH⁻. Millimoles of a species is M × V(mL) multiplied by how many H⁺ or OH⁻ each formula unit gives.

   n_(acid) × M_(acid) × V_(acid) = n_(base) × M_(base) × V_(base)

2. Count OH⁻ from base. Each M(OH)₂ gives 2 OH⁻, so its acidity is 2. Millimoles of OH⁻ = 2 × 0.05 × 30 = 3.
3. Count H⁺ from acid. Millimoles of acid = 0.1 × 10 = 1. If its basicity is n, it gives n × 1 = n millimoles of H⁺.
4. So. Setting n = 3 balances the two sides, so acid A is tribasic and its basicity is 3.

The common slip: 1.5. Forgetting that M(OH)₂ releases two OH⁻ per formula unit gives a basicity of 1.5, which is not even a whole number.

The idea to keep. In acid–base titrations, always multiply moles by basicity or acidity before equating. You are matching H⁺ to OH⁻, not molecule to molecule.

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