Alkalinity of water, in equivalent/litre (eq/litre), is given by \{ {HCO}_{3}{ }^{-}\}+2\{ {CO}_{3}{ }^{2-}\}+\{ {OH}^{-}\}-\{ {H}^{+}\} Where, { }represents concentration in mol/litre. For a water sample, the concentrations of {HCO}_{3}{ }^{-}=2 \times 10^{-3} {mol} / litre, {CO}_{3}{ }^{2-}=3.04 \times 10^{-4} {mol} / litre and the {pH} of water =9.0 . The atomic weights are: {Ca}=40 ; {C}=12 ; and {O}=16. If the concentration of {OH}^{-} and {H}^{+} are NEGLECTED, the alkalinity of the water sample (in {mg} /litre as {CaCO}_{3} ), is

GATE Civil Engineering (CE) 2020 Shift 2 Solved Paper

Question : 63 of 65

Marks: +1, -0

Alkalinity of water, in equivalent/litre (eq/litre), is given by

\{ {HCO}_{3}{ }^{-}\}+2\{ {CO}_{3}{ }^{2-}\}+\{ {OH}^{-}\}-\{ {H}^{+}\}

Where, { }represents concentration in mol/litre. For a water sample, the concentrations of

{HCO}_{3}{ }^{-}=2 × 10^{-3} {mol} /

litre,

{CO}_{3}{ }^{2-}=3.04 × 10^{-4} {mol} /

litre and the

{pH}

of water

=9.0 .

The atomic weights are:

{Ca}=40 ; {C}=12 ;

and

{O}=16

. If the concentration of

{OH}^{-}

and

{H}^{+}

are NEGLECTED, the alkalinity of the water sample (in

{mg}

/litre as

{CaCO}_{3}

), is

Solution:

We know

{HCO}_{3}^{-}=2 × 10^{-3} {mol} {/}ℓ

{CO}_{3}^{2-}=3.04 × 10^{-4} {mol}{/}ℓ

According to question neglecting

{OH}^{-}

and

{H}^{+}

,
Alkalinity of water

=(2 × 10^{-3}+2 × 3.04 × 10-4

=2.608 × 10-3

of eq

{/}ℓ

In terms of

{caco}_{3}

Alkalinity of water

=2.608 × 10^{-3} × 50 {gm} {/} ℓ

as caco

=130.4 {mg} {/} ℓ

{caco}_{3}

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