RH 2 (ion exchange resin) can replace Ca 2 + ions in hard water as: RH 2 + Ca 2 + ⟶ RCa + 2H + If 1 L of hard water after passing through RH 2 has pH = 3, then hardness in parts per million of Ca 2 + is :

Step-by-step solution

[ H + ] =10 - 3 M [ ∵ pH = 3 = - log [ H + ] =10 - 3 moles per litre [ Ca 2 + ] = 5 × 10 - 4 moles per litre = 5 × 10 - 1 moles of Ca 2 + per 10 3 L = 5 × 10 - 1 × 40 g Ca 2 + per 10 6 g of H 2 O Ca 2 + = 20 ppm = (H).