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CH301 - Type2.acid

# CH301 - Type2.acid - page 1 TYPE 2(acid buffer both HA and...

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TYPE 2 (acid buffer) ( both HA and A - are put into solution ) For a weak acid (HA) in water: HA + H 2 O H 3 O + + A - For a salt, NaA, of the weak acid (HA) in water: NaA (s) + H 2 O Na + + A - (100% ionization!!!) NOTICE: There are now TWO sources for the ion A - . When there is more than one source for an ion in solution, this is called the COMMON ION EFFECT. Please be VERY aware that the main source for the ion, A - , is from the salt and not from the acid. This is the key to understanding TYPE 2 problems. Here, the two reactions NaA (s) are shown intersecting + at the point of the H 2 O common ion. 100% ionization Na + + HA + H 2 O H 3 O + + The following equilibrium condition still applies: K a = [H + ][A ± ] [HA] now let C HA be the analytical concentration (label) of the weak acid (HA) and C A - be the concentration of the salt of the weak acid (NaA, or a better term, the conjugate base concentration, [A - ] ). Our Equilibrium "ICE" Table [HA] [H + ] [A - ] initial C HA 0 C A - change -x -x -x equilibrium C HA – x x C A - x K a = x C A ± + x ( ) C HA ± x ( ) Equation 2.1 K a C HA ± K a ( x ) = C A ±

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