1 Answers

Aman Bansal
592 Points
10 years ago

Dear Sumant,

Kw is essentially just an equilibrium constant for the reactions shown. You may meet it in two forms:

Based on the fully written equilibrium . . .

. . . or on the simplified equilibrium:


You may find them written with or without the state symbols. Whatever version you come across, they all mean exactly the same thing!

You may wonder why the water isn't written on the bottom of these equilibrium constant expressions. So little of the water is ionised at any one time, that its concentration remains virtually unchanged - a constant. Kw is defined to avoid making the expression unnecessarily complicated by including another constant in it.

The value of Kw

Like any other equilibrium constant, the value of Kw varies with temperature. Its value is usually taken to be 1.00 x 10-14 mol2 dm-6at room temperature. In fact, this is its value at a bit less than 25°C.

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Aman Bansal

Askiitian Expert


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