Question

A sample of water is found to contain \(\mathrm{F^-}\) (1 ppm), \(\mathrm{SO_4^{2-}}\) (>500 ppm), \(\mathrm{NO_3^-}\) (40 ppm). Which ion(s) make(s) the water sample unfit for drinking?

• A. \(\mathrm{F^-}\) & amp; \(\mathrm{NO_3^-}\)
• B. \(\mathrm{SO_4^{2-}}\)
• C. \(\mathrm{NO_3^-}\)
• D. \(\mathrm{F^-}\)

Hint:

High sulphate concentration \[ \boxed{(>500\ \text{ppm})} \] can make drinking water unsuitable, whereas \[ \boxed{ \mathrm{F^-}=1\ \text{ppm} } \] and \[ \boxed{ \mathrm{NO_3^-}=40\ \text{ppm} } \] are within acceptable limits.

Answer 1

The Correct Option is B

Step 1: Recall the permissible limits in drinking water.& nbsp;

The desirable limits are approximately:

\[ \begin{aligned} \mathrm{F^-} & amp;\le 1\text{--}1.5\ \text{ppm},\\ \mathrm{NO_3^-} & amp;\le 45\ \text{ppm},\\ \mathrm{SO_4^{2-}} & amp;\le 200\ \text{ppm}. \end{aligned} \]

Sulphate concentration above

\[ 500\ \text{ppm} \]

makes water unsuitable for drinking.

Step 2: Compare the given values.

\[ \begin{aligned} \mathrm{F^-} & amp;= 1\ \text{ppm} & amp;& amp;\text{(acceptable)},\\ \mathrm{NO_3^-} & amp;= 40\ \text{ppm} & amp;& amp;\text{(acceptable)},\\ \mathrm{SO_4^{2-}} & amp;> 500\ \text{ppm} & amp;& amp;\text{(exceeds the permissible limit).} \end{aligned} \]

Step 3: Choose the correct option.

Hence,

\[ \boxed{\mathrm{SO_4^{2-}}} \]

makes the water sample unfit for drinking.

Therefore, the correct option is \[ \boxed{(B)}. \]