GATE 2026 ES – Question 41
A water sample has $Na^+,Ca^{2+},Mg^{2+},Cl^-,SO_4^{2-},NO_3^-$ concentrations 30,10,15,30,60,5 mg/L respectively. Atomic weights (g/mol) are Ca 40,Mg 24,Na 23,Cl 35.5,S 32,O 16,N 14. Find hardness in mg/L as $CaCO_3$.
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Correct answer: (A) 87.5
Explanation
Hardness here is due to calcium and magnesium. Convert each concentration to CaCO₃ equivalents using equivalent weight 50: calcium contributes $10(50/(40/2))=25$ mg/L and magnesium contributes $15(50/(24/2))=62.5$ mg/L. Total hardness is $25+62.5=\mathbf{87.5}$ mg/L as CaCO₃,**A**. Sodium and the listed anions do not add to hardness.