Q10 / 73 · NTA Abhyas (JEE Main) 2020 · Chemistry · Some Basic Concepts of Chemistry

RH2 (ion exchange resin) can replace Ca2+ ions in hard water as:

RH2 + Ca2+⟶ RCa + 2H+

If 1 L of hard water after passing through RH2 has pH = 3, then hardness in parts per million of Ca2+ is :

  • A
    10 ppm
  • B
    40 ppm
  • C
    100 ppm
  • Correct answer
    D
    20 ppm

Solution

[ H+ ] =10 - 3 M [ ∵ pH = 3 = - log [ H+ ] 

         =10 - 3 moles per litre

[ Ca2 + ] = 5 × 10 - 4 moles per litre

         = 5 × 10 - 1 moles of Ca2 + per 103 L

         = 5 × 10 - 1× 40 g Ca2 + per 106 g of H2O

Ca2 + = 20 ppm = (H).

Hence, the correct answer is option: 'D'.

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