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Full set of Notes with Fill-Ins - San Jose State University

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Chem 155 Unit 10 Page 186 <strong>of</strong> 316Deviations due to Shifting Equilibria:For many molecules, especially weak acids, only one <strong>of</strong> theconjugate acid / base pair may absorb at a givenwavelength.This is how acid-base indicators work!Suppose only the conjugate base <strong>of</strong> a weak acid is colored.(think phenolphthalein!) What would a Beer’s law plot <strong>of</strong> anindicator like phenolphthalein (HA) in pure water look like?HA ⇔ H + + A -1. K EQ = [H + ][A - ] / [HA]absorbingGod, please just dump this one, please!Vastly simplify or dump.Concentration <strong>of</strong> HA insolution, theundissociated fraction.2. Conservation <strong>of</strong> mass: [HA 0 ] = [HA] + [A - ]so: [HA] = [HA 0 ] – [A - ]Number <strong>of</strong> moles <strong>of</strong> HA4. [A - ] = [H + ] (if >> 10 -7 M) divided by volume <strong>of</strong> insolution. Total or formalconcentration.5 = 1 + 4 K EQ = [A - ][A - ] / [HA]6 = 5 + 2 K EQ = [A - ] 2 / [HA 0 ] – [A - ]Page 186 <strong>of</strong> 316

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