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(A) \[{10^{ - 14}}\]

(B) \[{10^{ - 12}}\]

(C) \[2 \times {10^{ - 6}}\]

(D) \[2 \times {10^{ - 7}}\]

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\[{K_W} = [{H^ + }][O{H^ - }]\]

Due to the electrical conductivity of water, it undergoes dissociation. Water being a weak electrolyte undergoes only partial dissociation. Dissociation of pure water to a small extent into \[{H^ + }\] and \[O{H^ - }\] ions by itself is called self-dissociation.

Equilibrium reaction of the pure water is given below:

\[{H_2}O \rightleftharpoons {H^ + } + O{H^ - }\]

Dissociation constant of the acid is

\[K = \dfrac{{[{H^ + }][O{H^ - }]}}{{{H_2}O}}\]……. (1)

Concentration of the unionized ions is the same as the starting concentration.

\[{\text{[}}{{\text{H}}_{\text{2}}}{\text{O] = constant}}\]

Therefore, equation (1) becomes

\[{K_W} = [{H^ + }][O{H^ - }]\]…… (2)

Equation (2) gives ionic products of water.

Ionic product of pure water at \[{25^o}C\] is

\[{K_W} = [{H^ + }][O{H^ - }] = 1 \times {10^{ - 14}}\]……... (3)

In pure water, hydrogen ion concentration \[[{H^ + }]\] is equal to the hydroxide ion concentration \[[O{H^ - }]\].

i.e. \[[{H^ + }] = [O{H^ - }]\]

The concentration of each ion can be calculated from the equation for the ionic product of water.

We can take the concentration of \[[{H^ + }]\] and \[[O{H^ - }]\]as \[x\].

Substituting \[x\] into equation (3) gives

\[{K_W} = [{H^ + }][O{H^ - }] = 1 \times {10^{ - 14}}\]

\[{K_W} = {x^2} = 1 \times {10^{ - 14}}\]

\[x = 1 \times {10^{ - 7}}\]

Therefore, \[[{H^ + }] = [O{H^ - }] = 1 \times {10^{ - 7}}\]

Hence, product of \[{H^ + }\] and \[O{H^ - }\] ions concentration at room temperature is

\[[{H^ + }][O{H^ - }] = 1 \times {10^{ - 7}} \times 1 \times {10^{ - 7}} = 1 \times {10^{ - 14}} = {10^{ - 14}}\]

Let us try to understand the pH value of acid, basis and neutral solution.

-Acidic solution will have pH value less than 7.

-Basic solution will have pH value more than 7.

-neutral solution will have pH value equal to 7.

- In basic solution, hydroxide ion concentration is greater than hydrogen ion concentration.

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