10.0 dm³ of water was added to 2.0 mol dm⁻³ of a 2.5 dm³ solution of HCl. What is the concentration of the final solution (mol dm⁻³)?

Correct answer: 0.4 mol dm⁻³

Explanation: Use dilution: \(C_1V_1=C_2V_2\).
\(C_1=2.0\ \text{mol dm}^{-3}\), \(V_1=2.5\ \text{dm}^3\). Moles of HCl \(=2.0\times2.5=5.0\ \text{mol}\).
Water added \(=10.0\ \text{dm}^3\) → final volume \(V_2=2.5+10.0=12.5\ \text{dm}^3\).
\(C_2=\dfrac{5.0}{12.5}=0.4\ \text{mol dm}^{-3}\).

• 0.4 — Correct.
• 8.0 — Too large; dilution must lower concentration.
• 2.0 — Unchanged; ignores dilution.
• 0.5 — Close but incorrect; arises from wrong final volume.