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Water acts as both acid and base
Dissociates into H3O+ and OH- ions
2H2O(l) H3O+(aq) + OH-
(aq)
Rewritten, H2O(l) + H2O(l) H3O+
(aq) + OH-(aq)
Water Dissociation
Rate of reaction reaches equilibrium and allows [H3O+] and [OH-] to be determined
2H2O(l) H3O+(aq) + OH-
(aq)
Kw = [H3O+] [OH-] Kw = water dissociation constant = 1x10-14for
pure water [H3O+] = 1x10-7 M in pure water [OH-] = 1x10-7 M in pure water
Water Dissociation (cont.)
Kw = [H3O+] [OH-] = 1 x 10-14 regardless of
where ions come
More acid, increase [H3O+] /decrease [OH-] 2 sources for [H3O+] , so less water dissociation
More base, decrease [H3O+] /increase [OH-]
Acid/Base Addition
Measure of the concentration of [H3O+] ions in an
acidic/basic solution Uses logarithmic scale
pH = -log[H+] or –log[H3O+]
[H3O+] = 10-pH
pOH = -log[OH-]
pH + pOH = 14
pH
pH Scale
Logarithmic scale. Measures the concentration of
hydrogen ions [H+] in a solution. Range from 0-14. NEUTRAL, pH=7. (pure water) BASE, pH > 7. (ocean water, milk of
magnesia, baking sodea) ACID, pH < 7. (stomach acid/HCl,
vinegar, soft drinks)
Increase [H3O+], decrease pH value, decrease [OH-]
Decrease [H3O+], increase pH value, increase [OH-]
pH scale (cont.)
How do we measure the pH of a solution?
Acid-base indicators Weak acids/bases (ex. litmus paper)
pH meter
Strong acids completely dissociate in solution
SOOO solution’s [H3O+] ~ [H3O+] in acid All [H3O+] ions resulting from acid
pH and strong acids
1) [H3O+] = 1x10-7 M in water
not a big number
2) LeChatlier’s Principle As more H3O+ ions dissociate from water, reaction
shifts to LEFT to compensate If more H3O+ ions added to solution, water reforms to
try and compensate SOOO decrease in [H3O+] from water dissociation
Why is the [H3O+] from water dissociation not a
factor?