CH302 Chapter 7 notes part 2

CH302 Chapter 7 notes part 2 - M acetic acid we looked at...

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CH302 Chapter 7 notes part 2 ACIDS and BASES pH of a WEAK Acid In solution we have two potential acids: HA (aq) H + (aq) + A - (aq) H 2 O (l) H + (aq) + OH - (aq) BOTH species can provide H + but K w is often MUCH smaller than K a so we assume HA is the strongest acid. [ ][ ] [] HA A O H K 3 a + = Example: Finding pH for a Weak Monoprotic Acid Calculate the concentrations of the various species in 0.15 M acetic acid, CH 3 COOH, solution. Percent Dissociation Percent Dissociation = amount dissociated(mol/L) x 100% initial concentration(mol/L) Another way to see how strong an acid is. A STRONG acid will be 100% dissociated. Example: In a 0.12 M solution of a weak monoprotic acid, HY, the acid is 5.0% ionized. Calculate K a for this acid. Using Ka to get pH, the % dissociation: The pH of a 0.10 M solution of a weak monoprotic acid, HA, is found to be 2.97. What is the value for K a ? What is the percent ionization?
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Now calculate the percent ionization for the 0.15
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Unformatted text preview: M acetic acid we looked at earlier. Calculate the concentrations of the species in 0.15 M hydrocyanic acid, HCN, solution. K a = 4.0 x 10-10 for HCN We'll fill this out and compare them: 0.15 M HCN 0.15 M acetic acid % pH [H + ] K a Solution Le Chatelier's Principle applied to Dilution of Weak Acids [ ][ ] [ ] HA A O H K 3 a + = Add enough water so that [ ][ ] [ ] 10 / K 10 HA / 10 / A 10 / O H Q a 3 = = + all concentrations decrease by a factor of 10: System will move to the RIGHT: Acid will dissociate further! As Q increases back to be equal to K a , the % dissociation increases! K a vs. % Dissociation Although BOTH indicate if an acid is strong or weak: K a is CONSTANT for constant T no matter the dilution : % dissociation will INCREASE as dilution increases: See Fig 7.5...
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CH302 Chapter 7 notes part 2 - M acetic acid we looked at...

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