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Chpt%2018-2

# Chpt%2018-2 - Weak Acids/Bases 18.4 18.6 Last time General...

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Weak Acids/Bases 18.4 – 18.6

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Last time General equations: HA( aq ) + H 2 O( l ) H 3 O + ( aq ) + A - ( aq ) K a For weak bases B( aq ) + H 2 O( l ) BH + ( aq ) + OH - ( aq ) K b Strong bases are usually BOH B + + OH - Weak acids and bases disassociate only partly, they reach equilibrium
Weak acids Consider the weak acid: HCO 2 H K a = 1.8*10 -4 If someone dissolves 4.8 g of HCO 2 H in 85 mL of water. What will be the pH of the water? Things you’ll need: Disassociation reaction Equilibrium Equation Equilibrium Table

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Percent disassociation [ ] [ ] initial dissoc HA HA d Dissociate Percent = Solve for the percent dissociated for: HCO 2 H K a = 1.8*10 -4 If the initial concentration is 1.227 M (same as previous problem at start of class) If the initial concentration is 0.100 M
More Equilibrium If 0.85 M HClO 2 solution has a pH of 1.04, what is the K a for HClO 2 You can check your answer with the appendix in the back

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Polyprotic acids Conceptually: Poly means many: H 3 PO 4 , H 2 SO 4 Multiple dissociation reactions: H 2 SO 4 + H 2 O H 3 O + + HSO 4 - K a1 HSO
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