Chp15,Acid-Base_Calc

Chp15,Acid-Base_Calc - Chang 7th Edition Chapter 15...

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Chang, 7 th Edition, Chapter 15, Worksheet #1 S. B. Piepho, Fall 2002 Acid-Base Calculations The Ion-Product Constant for Water, K w Water undergoes ionization to a small extent: H 2 0( l ) H + ( aq ) + OH ( aq ) The equilibrium constant for the reaction is the ion-product constant for water K w : K w = [H + ][OH - ] =1.0 ´ 10 - 14 (1) This is a key equation in acid-base chemistry. Note that the product of [H + ] and [OH ] is a constant at a given temperature (Eq(1) value is for 25 o C). Thus as the hydrogen ion concentration of a solution increases, the hydroxide ion concentration decreases (and vice versa). The pH scale is widely used to report the molar concentration of hydrogen ion H + ( aq ) in aqueous solution. The pH of a solution is defined as pH = - log 10 [H + ] (2) Similarly, pOH and p K w are defined as pOH = - log 10 [OH - ] (3) p K w = - log 10 ( K w ) =14.00 (4) If you take the log 10 of both sides of Eq(1), multiply the resulting equation by (-1), and use the definitions of pH, pOH and p K w above, the result is the very useful equation pH + pOH = p K w = 14.00 (5) Equations (2) and (3) above may be solved for [H + ] and [OH ] respectively to give [H + ] =10 - pH (6) [OH - ] =10 - pOH (7) (Here we use the well known rule that if log 10 y = x , then y =10 x .) In practice, the pH scale is only used when [H + ( aq )] is less than 1.0 M. Acidic, basic, and neutral solutions can be distinguished as shown below: Type of Solution pH [H + ] Color of litmus Acidic < 7.00 > 1.0 × 10 - 7 pink Neutral = 7.00 = 1.0 × 10 - 7 in between Basic > 7.00 < 1.0 × 10 - 7 blue Page 1 of 6

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Chang, 7 th Edition, Chapter 15, Worksheet #1 S. B. Piepho, Fall 2002 pH and [H + ] Calculations for Strong Acids and Bases By definition, strong acids and bases are 100% ionized in water solution. Ionization of a strong acid gives rise to H + ions, and ionization of a strong base produces OH ions. The equilibrium constant for a strong acid or strong base is undefined , since the reaction the ionization is complete. There is no equilibrium! In nearly all cases of practical interest the [H + ] for a strong acid (or the [OH ] for a strong base) is determined completely by the stoichiometry of the reaction . Once the [OH ] or pOH is known for a base, the [H + ] or the pH of the base may be calculated using Eq(1) and/or
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Chp15,Acid-Base_Calc - Chang 7th Edition Chapter 15...

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