Consider the reaction represented by the following equation: Na2CO3(aq) + 2HCl(aq) 2NaCl(aq) + H2O(l) + CO2(g)…

Analyze the question: We need the volume of Na2CO3 solution (0.02 mol/dm³) that exactly reacts with 40 cm³ of 0.10 mol/dm³ HCl.

Concept: Use the mole ratio from the balanced equation: Na2CO3 : HCl = 1 : 2

Solution:

1. Moles of HCl = concentration × volume (in dm³) = 0.10 × (40/1000) = 0.004 mol

2. From the equation, 2 mol HCl reacts with 1 mol Na2CO3, so moles of Na2CO3 = 0.004 / 2 = 0.002 mol

3. Volume of Na2CO3 = moles / concentration = 0.002 / 0.02 = 0.1 dm³ = 100 cm³

Check with the formula: (CaVa)/(CbVb) = na/nb (0.10 × 40)/(0.02 × Vb) = 2/1 4 / (0.02 Vb) = 2 0.02 Vb = 2 Vb = 100 cm³ ✔

Answer: B. 100 cm³

Tip: Always convert to moles first, then apply the mole ratio from the balanced equation. A common mistake is to forget the 1:2 ratio, which gives 200 cm³ (option A) if you divide the moles incorrectly, or 40 cm³ if you ignore the ratio altogether. The ratio matters.