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AP Ch 16 Acid Base Equilibria

AP Ch 16 Acid Base Equilibria - Acid-Base Equilibria...

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Acid-Base Equilibria Chapter 16
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The common ion effect is the shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance. The presence of a common ion suppresses the ionization of a weak acid or a weak base. Consider mixture of CH 3 COONa (strong electrolyte) and CH 3 COOH (weak acid). CH 3 COONa ( s ) Na + ( aq ) + CH 3 COO - ( aq ) CH 3 COOH ( aq ) H + ( aq ) + CH 3 COO - ( aq ) common ion 16.2
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Consider mixture of salt NaA and weak acid HA. HA ( aq ) H + ( aq ) + A - ( aq ) NaA ( s ) Na + ( aq ) + A - ( aq ) K a = [H + ][A - ] [HA] [H + ] = K a [HA] [A - ] -log [H + ] = -log K a - log [HA] [A - ] -log [H + ] = -log K a + log [A - ] [HA] pH = p K a + log [A - ] [HA] p K a = -log K a Henderson-Hasselbalch equation 16.2 pH = p K a + log [conjugate base] [acid]
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What is the pH of a solution containing 0.30 M HCOOH and 0.52 M HCOOK? HCOOH ( aq ) H + ( aq ) + HCOO - ( aq ) Initial ( M ) Change ( M ) Equilibrium ( M ) 0.30 0.00 - x + x 0.30 - x 0.52 + x x 0.52 + x Common ion effect 0.30 – x 0.30 0.52 + x 0.52 pH = p K a + log [HCOO - ] [HCOOH] HCOOH p K a = 3.77 pH = 3.77 + log [0.52] [0.30] = 4.01 16.2 Mixture of weak acid and conjugate base!
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A buffer solution is a solution of: 1. A weak acid or a weak base and 2. The salt of the weak acid or weak base Both must be present! A buffer solution has the ability to resist changes in pH upon the addition of small amounts of either acid or base. 16.3 Add strong acid H + ( aq ) + CH 3 COO - ( aq ) CH 3 COOH ( aq ) Add strong base OH - ( aq ) + CH 3 COOH ( aq ) CH 3 COO - ( aq ) + H 2 O ( l ) Consider an equal molar mixture of CH 3 COOH and CH 3 COONa
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HCl H + + Cl - HCl + CH 3 COO - CH 3 COOH + Cl - 16.3
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Which of the following are buffer systems? (a) KF/HF (b) KBr/HBr, (c) Na 2 CO 3 /NaHCO 3 (a) KF is a weak acid and F - is its conjugate base buffer solution (b) HBr is a strong acid not a buffer solution (c) CO 3 2- is a weak base and HCO 3 - is it conjugate acid buffer solution 16.3
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= 9.20
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