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GATE 2021 ES – Question 63

Environmental Chemistry · Principles of Water Chemistry · 2 marks · Numerical answer

Water containsNa⁺ .25 M, Ca²⁺ .12 M, Cl⁻ .32 M, HCO₃⁻ .05 M. Find ionic strength in M, to two decimals.

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Correct answer: .50 to .68

Explanation

Ionic strength is $I=\tfrac12\sum c_iz_i^2$. Thus I=.5[.25(1)+.12(4)+.32(1)+.05(1)]=.5(1.10)=**.55 M**. Calcium’s divalent charge is squared, not merely multiplied by 2. The listed analytical composition is used directly as requested.

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