The ion product of water ( {pK}_{ {w}}) is 14 . If a rain water sample has a {pH} of 5.6, the concentration of {OH}^{-} in the sample (in 10^{-9} {mol}/ litre, round off to one decimal place ), is__________

GATE Civil Engineering (CE) 2020 Shift 2 Solved Paper

Question : 42 of 65

Marks: +1, -0

The ion product of water

( {pK}_{ {w}})

is 14 . If a rain water sample has a

{pH}

5.6

, the concentration of

{OH}^{-}

in the sample (in

10^{-9} {mol}/

litre, round off to one decimal place ), is__________

Solution:

{pH}+ {POH}=14

⇒ 5.6+ {POH}=14

⇒ {POH}=14-5.6=8.4

\;\text " Also, "\; {POH}=-\log [ {OH}^{-}]

⇒ 8.4=-\log [ {OH}^{-}]

⇒ 10^{-8.4}=[ {OH}^{-}]

⇒[ {OH}^{-}]=3.981 × 10^{-9} {M}

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