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GATE 2020 CE (CE2) – Question 62

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

Water has $pK_w=14$. Rainwater pH is 5.6. Find OH⁻ concentration in units of $10^{-9}$ mol/L, to one decimal.

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Correct answer: 3.8 to 4.1

Explanation

$pOH=14-5.6=8.4$, so $[OH^-]=10^{-8.4}=3.98107\times10^{-9}$ mol/L. In the requested units the answer is **4.0**.