3. Methods for Calculating the pH of an Aqueous Solution With a strong acid or base – we assume 100% dissociation and directly get the concentration of H 3 O + or OH ¯ to get the pH. But what happens when we are dealing with weak acids or bases? To answer this question we need to apply the ideas we have learned about chemical equilibrium.
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5. Equilibrium Constants for Weak Acids Weak acid has K a < 1 Leads to more [H 3 O + ] than in pure water and a pH of 2 - 7 Acid Ionization Constant In general:
7. Equilibrium Constants for Weak Bases Weak base has K b < 1 Leads to more [OH ¯ ] than in pure water and a pH of 7-12 Base Ionization Constant Write the ionization equation and the ionization constant for : H 2 PO 4 - (acid) and C 6 H 5 NH 2 (base) .
13. Calculate the pH of a 0.0010 M solution of formic acid, HCOOH . K a = 1.8 x 10 -4
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17. Calculate the pH of a 0.15 M solution of (CH 3 ) 3 N. K b = 6.3 x 10 -5
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24. Acid-Base Properties of Salts Assume 0.10 - x ≈ 0.10, because 100•K b < C o x =[HCO 3 - ] =[OH - ] = 0.0046 M Step 2 . Solve the equilibrium expression Step 3. Calculate the pH [OH - ]= 0.0046 M, pOH= - log [OH - ] = 2.34 pH + pOH = 14, so pH = 11.66
25. Calculate the pH of a 0.15 M solution of KNO 2 . K a (HNO 2 ) = 4.6 x 10 -4
26. Polyprotic Acids Some acids donate more than one H + per molecule: H 2 SO 4 , H 2 CO 3 , H 3 PO 4 These acids donate their protons in an stepwise manner: H 3 PO 4 (aq) + H 2 O(l) H 3 O + (aq) + H 2 PO 4 - (aq) K a =7.5 x 10 -3 H 2 PO 4 -(aq) + H 2 O(l) H 3 O + (aq) + HPO 4 2- (aq) K a =6.2 x 10 -8 HPO 4 2- (aq) + H 2 O(l) H 3 O + (aq) + PO 4 3- (aq) K a =3.6 x 10 -13 The resulting acids are weaker
27. Molecular structure and acid/base strength There are four main effects that influence the relative strength of an acid: Size of the anion Electronegativity of the H-bearing atom Inductive effect Resonance stabilization of the anion The first two factors influence binary acids The latter two factors influence non-binary acids
28. Binary Acid Strength Factors affecting binary acid strength The polarity of the H-A bond (most important factor when comparing atoms in the same row in the periodic table) electronegativity The bond energy of the H-A bond (dependent on the length of the bond; important when comparing atoms in the same column in the periodic table) size of the anion HA type of acid
29. Binary Acid Strength Which of these is a stronger acid: HCl or HBr? Why? Which of these is a stronger acid: H 3 P or H 2 S? Why?
30. Strength of Non-binary Acids For nonbinary acids – Atoms with higher electronegativity can draw the electrons away from the bond with the acidic hydrogen – making that bond more polar. This is known as the inductive effect . Or if the anion of the acid has several equal resonance structures – the anion would be rather stable – and the acid more likely to lose its acidic hydrogen. This is known as resonance stablization .
31. Strength of Oxyacids Acids in which the acidic hydrogen is bonded directly to oxygen in an H-O-Z bond are called oxyacids. For each of the following pairs, which acid is stronger – and why? Strength?
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34. Weak Bases: Amines Amines are compounds that, like ammonia, have a nitrogen atom with three of its valence electrons in covalent bonds and an unshared electron pair on the nitrogen atom. The lone pair of electrons can accept an H + . H 2 O H +
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36. 1. What is the pH of a solution of 0.050 M (C 2 H 5 ) 2 NH?
37. 2. What is the pH of a solution of 0.050 M HClO 4 ?
38. Summary Activity 3. Label the following as acidic, basic, or neutral solutions: KCl NaNO 2 (CH 3 ) 2 NH 2 Cl
39. Which of the following is the strongest acid? HClO 2 , HBrO 2 , HIO 2
40. Which of the following is the strongest acid? H 3 N, H 2 O, HF
