Sodium oxide reacts with water to produce sodium hydroxide. 20.0 g of sodium oxide is dissolved in 500 mL of water. Neglecting the change in volume, the concentration of the resulting NaOH solution is ______________ × 10⁻¹ M. (Nearest integer) [Atomic mass : Na = 23.0, O = 16.0, H = 1.0]

Solution

Chemistry ma’am · 4 steps

One Na₂O gives two NaOH, so double the moles before you divide by the volume.

1. Balanced equation. Na₂O + H₂O → 2NaOH. Each mole of sodium oxide gives 2 moles of sodium hydroxide.
2. Moles of Na2O. Molar mass of Na₂O = 2 × 23.0 + 16.0 = 62.0 g mol⁻¹. So 20.0 g is 20.0/62.0 = 0.3226 mol.
3. Moles of NaOH. Moles of NaOH = 2 × 0.3226 = 0.6452 mol. The volume stays 500 mL = 0.500 L.

   M = (0.6452 mol)/(0.500 L) = 1.29 M = 12.9 × 10⁻¹ M

4. So. The concentration is 12.9 × 10⁻¹ M, which rounds to 13.

The common slip: 6. Forgetting that one Na₂O gives two NaOH gives 0.645 M, that is 6, not 13.

The idea to keep. Always go through the balanced equation: convert grams of what you have into moles of what you want, then divide by litres of solution.

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