A 100 L sample of hard water, having only temporary hardness is dried to obtain 20 mg of CaCO₃ as residue. Which of the following may be true for the sample of water? (Density = 1 g/ml)

Mol. mass of CaCO₃ = 100 Mol. mass of Ca = 40 20 mg of CaCO₃ has ((40*20) / 100) = 8 mg of Ca⁺² ions Since, 8 mg of Ca⁺² in 100 L sample of hard water. 10*1000 ml sample of hard water has 8 mg of Ca⁺² therefore, 10^6 ml sample of hard water has 80 mg of Ca⁺²,i.e., 80 ppm, which is the required solution of the given problem. Hence, this is a correct option.