GATE 2025 CE (CE2) – Question 61

Water and Waste Water Quality and Treatment · Water quality standards, parameters and water quality index · 2 marks · Numerical answer

The analyses results of a water sample are given below. The non-carbonate hardness of the water (in mg/L) as CaCO$_3$ is ______ (in integer).

Ca$^{2+}$ = 150 mg/L as CaCO$_3$
Mg$^{2+}$ = 40 mg/L as CaCO$_3$
Fe$^{2+}$ = 10 mg/L as CaCO$_3$
Na$^+$ = 50 mg/L as CaCO$_3$
K$^+$ = 10 mg/L as CaCO$_3$
CO$_3^{2-}$ = 120 mg/L as CaCO$_3$
HCO$_3^-$ = 30 mg/L as CaCO$_3$
Cl$^-$ = 50 mg/L as CaCO$_3$; Other anions were not analysed.

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: 50

Explanation

The hardness is caused by the divalent cations: $150 + 40 + 10 = 200$ mg/L as CaCO₃. The alkalinity is $120 + 30 = 150$ mg/L as CaCO₃. The carbonate hardness is the smaller of the total hardness and the alkalinity, 150 mg/L, so the non-carbonate hardness is $200 - 150 = 50$ mg/L as CaCO₃.