lec24 - Last Time Titrations Weak acid/strong base Strong...

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1 Last Time • Titrations – Weak acid/strong base – Strong acid/weak base Determination of K a HA(aq) + OH (aq) A (aq) + H 2 O(l) • At stoichiometric point, n OH = initial n HA = n A • Halfway to the stoichiometric point –n OH = n A = n HA = initial n HA /2 – using total volume : [HA] = [A ] Ka = [H 3 O + ] or pK a = pH
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2 pH Indicators • Weak acids that change color depending on pH HIn(aq) + H 2 O(aq) H 3 O + (aq) + In (aq) • One color when an acid HIn and another when a conjugate base In • Endpoint when [HIn] = [In ] giving pK In = pH • Color change occurs when pH = pK In Buffers • Solutions that resist pH changes when acids or bases are added – Example: plateau 1/2 way to S – point • Either a weak acid and conjugate base or weak base and its conjugate acid – Base accepts protons to resist added acid – Acid donates protons to resist added base – Typically weak acid or base and its salt in equal concentration
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3 Buffer Preparation
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lec24 - Last Time Titrations Weak acid/strong base Strong...

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