20 cm3 of 0.09 mol dm3 solution of tetraoxosuiphate (Vi) acid requires 30cm3 of sodium hydroxide solution for…

Analyze the question: This is an acid-base titration. H₂SO₄ (tetraoxosulphate(VI) acid) is dibasic and is neutralized by NaOH.

Concept: Use the balanced equation and the titration formula:

C_a V_a / (C_b V_b) = n_a / n_b

Balanced equation: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

So n_a = 1 and n_b = 2.

Data:

• Acid: C_a = 0.09 mol/dm³, V_a = 20 cm³
• Base: C_b = ?, V_b = 30 cm³

Solution:

(0.09 × 20) / (C_b × 30) = 1/2

1.8 / (30 C_b) = 1/2

30 C_b = 3.6

C_b = 0.12 mol/dm³

Check with moles:

• Moles of H₂SO₄ = 0.09 × 20/1000 = 0.0018 mol
• Moles of NaOH needed = 2 × 0.0018 = 0.0036 mol
• Concentration = 0.0036 / 0.030 = 0.12 mol/dm³ ✔

Tip: Always write the balanced equation first. Forgetting the 2 in front of NaOH gives 0.06, and using the wrong ratio is the most common mistake in titration questions. For a dibasic acid like H₂SO₄, the base concentration is always twice as "strong" per mole, so multiply the acid's moles by 2.