 Acidity, Basicity, and pH

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Acidity, Basicity, and pH http://videos.howstuffworks.com/hsw/25353-compounds-ph-scale-and-indicators-video.htmvideos.howstuffworks.com/hsw/25353-compounds-ph-scale-and-indicators-video.htm

Self-Ionization of Water Water acts as both acid and base in same reaction Proceeds minimally to the right H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH - (aq) Pure H 2 O, 25°C, [H 3 O + ] & [OH - ]=1.0x10 -7 Concentration is small but significant K eq = [H 3 O + ][OH - ]/[H 2 O] 2

Self-Ionization of Water Pure water and dilute solutions [H 2 O] same-[H 2 O] moved to left side K eq [H 2 O] 2 =[H 3 O + ][OH - ] K w = [H 3 O + ][OH - ] ion product constant Pure water, 25°C, K w = 1.0 x 10 -14 K w = (1.0 x 10 -7 )(1.0 x 10 -7 ) Applies for all water solutions at 25°C, even when [H 3 O + ][OH - ] are not equal

Self-Ionization of Water Acidic, basic and neutral solutions all contain H 3 O + and OH - Acidic – [H 3 O + ] > 10 -7 Basic – [OH - ] > 10 -7 If the concentration of H 3 O + in blood is 4.0x10 -8 M, is blood acidic, basic, or neutral? What is the concentration of OH - ions in blood?

Self-Ionization of Water Blood is basic, [H 3 O + ] 4.0x10 -8 < 1.0x10 -7 K w = [H 3 O + ][OH - ] 1.0x10 -14 = (4.0x10 -8 )[OH - ] [OH - ] = 2.5x10 -7 M

Example What is the concentration of H 3 O + ions in a wheat flour and water solution if [OH - ] = 1.0x10 -8 M? Is wheat flour and water acidic, basic, or neutral? 1.0 x 10 -14 = [H 3 O + ](1.0 x 10 -8 M) [H 3 O + ] = 1.0 x 10 -6 M Acidic

pH Scale Based on logarithms pH = -log [H 3 O + ] Number’s base 10 logarithm is its exponent when 10 is the base, log (0.001) = 3 10 -3 = 0.001 1 unit change in pH is 10-fold change in concentration of H 3 O + Sig. Figs. – number of digits after decimal should equal number of sig. figs. In original number

pH In one brand of vegetable juice, the concentration of H 3 O + ions is 7.3 x 10 -5 M What is the pH of the juice? pH = -log [H 3 O + ] pH = - log (7.3 x 10 -5 ) Using log key = -4.13667714 pH = -(-4.13667714) = 4.14

Example In household bleach, the concentration of OH - ions is 5.0 x 10 -2 M. What is the pH? 1.0 x 10 -14 = [H 3 O + ](5.0 x 10 -2 ) [H3O+] = 2.0 x 10 -13 pH = -log (2.0 x 10 -13 ) pH = 12.70

Measuring pH Use acid-base indicator Indicator equation HIn (aq) + H2O (l) H3O+ (aq) + In- (aq) clearred Add acid, shift equilibrium to left –(litmus – HIn red) Add base, shift equilibrium to right –(litmus – In-blue)

Measuring pH Each indicator has a different equilibrium constant Different pH range over which color changes Select indicator depending on pH range