The pH scale and acid strength

Pure water consists mainly of undissociated molecules, H2O, although a small fraction of the molecules are dissociated, according to Equation 3.8 

At 25 °C the concentration of H+(aq) ions, which equals that of OH (aq) ions, is 1 X 10 mollitre The equilibrium constant for Equation 3.8 is given by  

In dilute aqueous solutions, the concentration of water is effectively constant. As K is constant, it follows that, at equilibrium, the product [H+(aq)][OH (aq)] will also be constant. From the special case of pure water, we know that this constant is 1 X 10 4moFlitre 2. So in dilute aqueous solutions at 25 °C, [H (aq)][OH (aq)] will always have the value 1 X 10 moFlitre .  

Consider a dilute solution of hydrochloric acid of concentration 0.1 mol litre k Note that the most concentrated hydrochloric acid is more than 100 times this value. In this dilute solution, [H+(aq)] = 0.1 mol litre , one million times the value in pure water. 

Now consider a dilute solution of sodium hydroxide of the same concentration. In this solution, the hydroxide ion concentration is [OH (aq)j = 1 x 10 mol litre . 

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