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GATE 2022 CE (CE1) – Question 43

Water and Waste Water Quality and Treatment · Water quality standards, parameters and water quality index · 2 marks · Multiple choice

In a water sample, the concentrations of Ca2+, Mg2+ and HCO3- are 100 mg/L, 36 mg/L and 122 mg/L, respectively. The atomic masses of various elements are: Ca = 40, Mg = 24, H = 1, C = 12, O = 16. The total hardness and the temporary hardness in the water sample (in mg/L as CaCO3) will be

  1. 400 and 100, respectively.
  2. 400 and 300, respectively.
  3. 500 and 100, respectively.
  4. 800 and 200, respectively.

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Correct answer: (A) 400 and 100, respectively.

Explanation

Convert divalent-ion concentrations to CaCO₃ equivalents: calcium contributes $100(50/20)=250$ mg/L and magnesium $36(50/12)=150$ mg/L. Total hardness = **400 mg/L**.

Bicarbonate alkalinity is $122(50/61)=100$ mg/L as CaCO₃. Temporary hardness is the lesser of total hardness and alkalinity: **100 mg/L**. **Answer: A**.