A solution of sodium trioxocarbonate (IV) contains 10.6g in 250cm3 of solution. Calculate the concentration of the solution. [Na2CO3 = 106.0]

Analyze the question: We need the concentration in mol dm⁻³ (molarity) of a Na₂CO₃ solution with 10.6 g dissolved in 250 cm³.

Concept: Molarity = number of moles ÷ volume in dm³ Number of moles = mass ÷ molar mass

Solution:

1. Moles of Na₂CO₃ = 10.6 g ÷ 106.0 g/mol = 0.1 mol

2. Convert the volume to dm³: 250 cm³ = 250 ÷ 1000 = 0.25 dm³

3. Concentration = 0.1 mol ÷ 0.25 dm³ = 0.4 mol dm⁻³

(Check: 0.1 mol in 250 cm³ means 0.4 mol in 1000 cm³, so 0.4 mol dm⁻³.)

Answer: A. 0.4 mol dm⁻³.

Tip: Always convert cm³ to dm³ by dividing by 1000 before calculating. A quick shortcut is that for 250 cm³, you multiply the moles by 4 to get the molarity. Wrong options like 1.0 or 25.0 come from forgetting this conversion.