Wastewater from a cement factory contains 0.280 g of Ca2 ion and 0.0550 g of Mg2 ion per 100.0 L of solution. The solution density is 1.001 g/mL. Calculate the Ca2 and Mg2 concentrations in ppm (by mass).

The correct answer is: 2.8 ppm Ca²⁺ and 0.55 ppm Mg²⁺

Explanation:

ppm (by mass) is equal to: mass solute (mg) / mass solution (kg)

First, we use the solution density to calculate the mass of 100.0 L solution in kg:

mass solution = 100.0 L x 1.001 g/ml x 1000 ml / 1L x 1kg/1000 g = 100.1 kg

Then, we divide the mass of each ion (in mg) into the mass solution to obtain the ppm:

mg Ca²⁺ = 0.280 g x 1000 mg/1g = 280 mg

mg Mg²⁺ = 0.0550 g x 1000 mg/1g = 55 mg

ppm Ca²⁺ = 280 mg Ca²⁺ / 100.1 kg solution = 2.797 mg Ca²⁺/kg solution = 2.8 ppm

ppm Mg²⁺ = 55 mg Mg²⁺ / 100.1 kg solution = 0.549 mg Mg²⁺/kg solution = 0.55 ppm