First, we write the equation for the dissolution of sodium carbonate.
Na₂CO₃(s) → 2Na⁺(aq) + CO₃⁻²(aq)
Next, we determine the moles of sodium carbonate that have been dissolved. The molar mass of sodium carbonate is 106 g/mol.
Moles = mass / Mr
Moles = 1.8 x 10⁻⁴ / 106
Moles = 1.70 x 10⁻⁶ moles
The moles of sodium ions will be double this, as observed in the equation, while the moles of carbonate ions will be equivalent to this.
The molarity of a substance is the number of moles per liter. The ppm of a substance is the milligrams of the substance per liter of water. Using these definitions,
Molarity Na₂CO₃ = 1.7 x 10⁻⁶ / 2.25 = 7.6 x 10⁻⁷ M
Molarity Na+ = 2 * 1.7 x 10⁻⁶ / 2.25 = 1.52 x 10⁻⁶ M
Molarity CO₃⁻² = 1.7 x 10⁻⁶ / 2.25 = 7.6 x 10⁻⁷ M
PPM Na₂CO₃ = 0.18 mg / 2.25 liters = 0.08 ppm
PPM Na⁺ = moles * molar mass in mg / liters
PPM Na⁺ = 2 * 1.7 x 10⁻⁶ * 23000 / 2.25 = 0.035 ppm
PPM CO₃⁻² = moles * molar mass in mg / liters
PPM CO₃⁻² = 1.7 x 10⁻⁶ * 60000 / 2.25 = 0.045 ppm
