Chem_215_Lecture_12_color - Indicators - a weak acid with a...

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1 Indicators - a weak acid with a conjugate base of different color HIn + H 2 O H 3 O + + In - ] HIn [ ] In ][ O H [ K 3 a + = Color 1 Color 2 Phenophthalein : HIn: Colorless In - :P i n k Basic- Small [H 3 O + ] Acidic- Large [H 3 O + ] 1 ] HIn [ ] In [ >> 1 ] HIn [ ] In [ << [In - ] << [HIn] Colorless [In - ] >> [HIn] Pink ] HIn [ ] In [ ] O H [ K 3 a + = 1 ] O H [ K 3 a << + 1 ] O H [ K 3 a >> +
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2 HIn + H 2 O H 3 O + + In - ] HIn [ ] In ][ O H [ K 3 HIn + = ] HIn [ ] In [ log pK pH 10 HIn + = Hi [H 3 O + ] Low pH ] O H [ K ] HIn [ ] In [ 3 HIn + = Indicator Color and pH Low [H 3 O + ] Hi pH When: pH = pK HIn : Intermediate color pH < pK HIn – 1 : Acid color pH < pK HIn + 1 : Base color
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3 HIn + H 2 O H 3 O + + In - ] HIn [ ] In ][ O H [ K 3 HIn + = ] HIn [ ] In [ log pK pH 10 HIn + = (Low pH) (High pH)
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4 Colors of Indicators depend upon pH
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5 Titration: Measurement of the number of moles of acid or base in a sample. Involves neutralization using known quantities of base or acid. Equivalence point: moles acid = moles base. Endpoint: point at which indicator undergoes color change.
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Chem_215_Lecture_12_color - Indicators - a weak acid with a...

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