lecture+g - Ch 11 Acids and bases Calculating the pH of...

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Ch 11 – Acids and bases Calculating the pH of acidic/basic solutions 2 – Aqueous weak acid or weak base solutions In the case of weak acids/bases, we’ll need to remember the dissociation does not favor the conjugate ions, but rather the un-ionized acids/bases, and we’ll need to use the value of K a or K b to determine [H 3 O + ] eq . Practice problem #20 Hydrofluoric acid has a K a of 7.2 × 10 4 . What is the pH of 3.44 M aqueous HF? Since we have K a , we know the reaction in question is: HF(aq) + H 2 O(l) H 3 O + (aq) + F (aq) K a = 7.2 × 10 4 I C E 3.44 N/A 10 7 (~0) 0 x N/A +x +x 3.44 x x x At equilibrium, we have: x 2 3.44 x = 7.2 × 10 4 ( 3.44) x 2 3.44 7.2 × 10 4 x = 0.050 M ([H + ] eq ) pH = log(0.05) = 1.30
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Ch 11 – Acids and bases Now that we know how to determine [H 3 O + ] eq for an acid, we can discuss another measure of how many ions have been produced by an acid, the PERCENT IONIZATION . This is defined as follows: % ionization = [H 3 O + ] eq [HA] 0 × 100% Some trends to keep in mind about percent ionization: - for the same [HA] 0 , stronger acids will have higher percent ionization. - more dilute solutions of the same acid will have higher percent ionization. The increase in percent ionization upon a simple dilution is actually an example of Le Châtelier’s principle. Before dilution, there are a certain number of particles (H + , A , and HA) floating around in a certain volume, establishing a certain particles to volume ratio. The number of particles is dependent on K a . After dilution, there are still the same number of particles as before but a larger volume. To adjust to this change in the particles to volume ratio, the system shifts to make more particles, which is towards ioniziation.
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Ch 11 – Acids and bases Practice problem #21 Calculate the pH of a aqueous solution containing 2.47 M ethylamine (C 2 H 5 NH 2 ) and 4.22 M pyridine (C 5 H 5 N), and determine the concentration of all aqueous phase ions present at equilibrium. We should first determine what reactions are present which will control the pH
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lecture+g - Ch 11 Acids and bases Calculating the pH of...

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