Question

A sample of ground water at a pH of 7.0 contains 122 mg/L of bicarbonates. What is the alkalinity of this water ?
$\text{(in terms of}CaC{O}_3)$

• A. 120 mg/L
• B. 60 mg/L
• C. 100 mg/L
• D. 200 mg/L

Answer 1

The correct option is C 100 mg/L

$\text{(c) Alkalinity expressed in equivalents}$

$=HC{O}_3^{-}+O{H}^{-}-H^{+}$

$\text{Milliequivalent mass of}\left[HC{O}_3^{-}\right]$

$=\frac{122}{61}=2meq/L$

$\text{Milliequivalent mass of}\left[O{H}^{-}\right]$

$=\frac{{10}^{-7}}{17}meq/L$

$\text{Milliequivalent mass of}\left[H^{+}\right]$

$=\frac{{10}^{-7}}{1}meq/L$

$\text{The milliequivalent of}\left[O{H}^{-}\right]\text{and}\left[H^{+}\right]$ $\text{are negligible. So Alkalinity as}CaC{O}_3$

$=2\times 50=100mg/L$