1. Write the balanced chemical equation:

2HCl(aq) + Ba(OH)₂(aq) → BaCl₂(aq) + 2H₂O(l)

2. Determine the mole ratio:

From the balanced equation, we see that 2 moles of HCl react with 1 mole of Ba(OH)₂.

3. Calculate the moles of Ba(OH)₂:

* Convert the volume of Ba(OH)₂ to dm³: 25.0 cm³ = 0.025 dm³

* Moles of Ba(OH)₂ = concentration × volume

* Moles of Ba(OH)₂ = 0.200 mol/dm³ × 0.025 dm³ = 0.005 mol

4. Calculate the moles of HCl required:

* Since the mole ratio is 2:1, we need twice the moles of HCl:

* Moles of HCl = 0.005 mol Ba(OH)₂ × (2 mol HCl / 1 mol Ba(OH)₂) = 0.010 mol HCl

5. Calculate the volume of HCl:

* Volume of HCl = moles of HCl / concentration of HCl

* Volume of HCl = 0.010 mol / 0.200 mol/dm³ = 0.050 dm³

6. Convert to cm³:

* Volume of HCl = 0.050 dm³ × (1000 cm³ / 1 dm³) = 50 cm³

Therefore, 50 cm³ of 0.200 mol/dm³ HCl is required to neutralize 25.0 cm³ of 0.200 mol/dm³ Ba(OH)₂.