An ion exchange resin, RH ₂ , can replace Ca ²⁺ in hard water as RH ₂+ Ca ²⁺ RCa ²⁺+2 H ⁺ When a 1.0 L hard water sample was passed through the resin, all H ⁺ ions were replaced by Ca ²⁺ ions and the pH of eluted water was found to be 2.0 . The hardness of water (as ppm of Ca ²⁺ ) in the sample of water treated is