CH301 - chapter 18 notes

# CH301 - chapter 18 notes - 1 CH 301 Chapter 18 The pH scale...

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1 CH 301 Chapter 18 The pH scale The Autoionization of Water We can write the autoionization of pure water as a dissociation reaction. The stoichiometry of the above equation indicates that the concentrations of the hydrogen ion (hydronium ions) and hydroxide ions in pure water must be equal. The double arrow indicates that this reaction is in equilibrium, that is, the concentration of hydrogen ions and hydroxide ions remain constant in pure water. Experimental measurements have determined that the concentration of each ion is 1.0 x 10 -7 M at 25 o C. Note that this is at 25 o C, not every temperature! We can determine the value of the extent that the hydronium (or H + ) ions and hydroxide ions form during autoionization by the following relationship: K w = [H 3 O + ][OH ] 1.0 x 10 14 = [H 3 O + ][OH ] This particular equilibrium constant is called the ion-product for water and given the symbol K w . Enclosing a chemical symbol in square brackets is one way to represent the molarity concentration (mol/L) of that chemical species. That is, [H 3 O + ] and [OH ] indicates the molarity concentration of each ion. Taking the square root of both sides of the equilibrium relationship indicates that the concentration of H 3 O + and OH are 10 –7 M. 14 14 2 14 7 1.0 10 [ ][ ] [ ] [ ] 1.0 10 1.0 10 1.0 10 H OH since H OH x x x M + + × = = × = × = × = In pure water, the [H + ] = 1.0 x 10 7 and the [OH ] = 1.0 x 10 7 . At 25.0 o C, these concentrations of hydrogen and hydroxide ions represent a neutral solution; the water is neither acidic nor basic. 2 ( ) 2 ( ) 3 ( ) ( ) 2 ( ) ( ) ( ) l l aq aq l aq aq H O H O H O OH or H O H OH + + → + + ← → + ←

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2 Slide 3 Table 18-2, p.747 When a substance is added to pure water that changes the concentration of either the [H + ] or [OH ], we can determine the
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