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GATE 2022 CE (CE1) – Question 42

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

Henry’s law constant for transferring O2 from air into water, at room temperature, is 1.3 mmol liter-atm . Given that the partial pressure of O2 in the atmosphere is 0.21 atm, the concentration of dissolved oxygen (mg/liter) in water in equilibrium with the atmosphere at room temperature is (Consider the molecular weight of O2 as 32 g/mol)

  1. 8.7
  2. 0.8
  3. 198.1
  4. 0.2

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Correct answer: (A) 8.7

Explanation

The supplied Henry coefficient is in mmol/(L·atm), so dissolved oxygen is $C=Hp=1.3(0.21)=0.273$ mmol/L.

Since O₂ has molar mass 32 g/mol, each mmol weighs 32 mg. Therefore $C=0.273(32)=8.736$ mg/L, or **8.7 mg/L (A)**.