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pH and Concentration

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pH Recall water H 2 O H + + OH - Every water molecule produces one hydrogen ion and one hydroxide ion At equilibrium, K eq = [products] [reactants] So for water: K eq = [H + ][OH - ] [H 2 O]

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Just for Water The concentration of pure water is constant, so: K eq = [H + ][OH - ] [H 2 O] K eq [H 2 O] = [H + ][OH - ] K w = [H + ][OH - ] K w is called the ion product constant for water At 298K, the [H + ] and [OH - ] are 1.0x10 -7 M So, K w is 1.0x10 -14 M 2

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Practice At 298K, the [H + ] is 1.0x10 -5 M. What is the [OH - ]? Is the solution acidic? Basic? Or neutral? K w = 1.0x10 -14 M 2 [H + ] = 1.0x10 -5 M K w = [H + ][OH - ] [OH - ] = K w /[H + ] = (1.0x10 -14 M 2 ) (1.0x10 -5 M) = 1.0x10 -9 M Since [H + ]>[OH - ], the solution is acidic

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Self-Practice Identify the solution as acidic or basic or neutral if the [OH - ] is 1.0x10 -2 M?

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So what? Of course, all that writing long numbers is annoying and confusing, so we invented pH! pH = -log[H + ] So, if [H + ] = 1.0x10 -3 then the pH is 3 In other words, the pH is the negative exponent of the ion’s concentration Since it represents the exponent, when pH changes by 1, it represents a 10 times change in concentration

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pH Scale

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