lecture+10sf-1

lecture+10sf-1 - pH of Strong Bases The most common...

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pH of Strong Bases The most common examples of strong bases are metallic hydroxides, most commonly hydroxides of group I and group II. Examples are NaOH, KOH, Ca(OH) 2 , Ba(OH) 2 . These compounds, when dissolved in water, dissociate completely into a metallic cation and the hydroxide ion. They are direct sources of hydroxide ion in aqueous solution. What is the pH of 0.10 M NaOH? Due to complete dissociation, [OH - ] = 0.10 M pOH = - log (0.10) = 1.00 pH = 13.00 What is the pH of 0.050 M NaOH? Due to complete dissociation, [OH - ] = 0.050 M pOH = - log (0.050) = 1.30 pH = 14 - 1.30 = 12.70 What mass of NaOH must be dissolved in a total of 300 mL of solution in order to produce a solution with pH 12.00? If pH = 12.00, then pOH = 2.00 Then [OH - ] = 10 -2.00 = 0.010 M 0.010 x 0.300 L x 40.0 = 0.12 g NaOH
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A saturated Mg(OH) 2 solution has pH 10.52. Calculate the molar solubility of Mg(OH) 2 . pH = 10.52 pOH = 14.00 - 10.52 = 3.48 [OH - ] = 10 -3.48 = 3.3 x 10 -4 M Mg(OH) 2 (s) → Mg 2+ (aq) + 2OH - (aq) Every mol of Mg(OH) 2 dissolving produces 2 mol OH - . An equivalent statement: for every mol of OH - produced, mol of Mg(OH) 2 dissolves. If [OH - ] = 3.3 x 10 -4 M, then the mol Mg(OH) 2 dissolved per liter = 2 1 (3.3 x 10 -4 ) M = 1.7 x 10 -4 M. This is the molar solubility of Mg(OH) 2 . pH of Weak Bases Weak acids are substances which partially donate a proton to water. The strength of the weak acid is measured by its K a value. HCN + H 2 O H 3 O + + CN - K a = 6.2 x 10 -10 acid base vacid base Weak bases are substances which partially accept a proton from water. The strength of the weak base is measured by its K b value. The prototypical example of a weak base is ammonia, NH 3 . The lone pair on the N atom serves as a site to which H + can attract. An ammonia molecule attracts H + from water. Water functions as the acid, and when water donates an H + ion to water, hydroxide ions form: NH 3 + H 2 O NH 4 + + OH - K b = 1.8 x 10 -5 base acid acid base
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3 . NH 3 + H 2 O NH 4 + +OH - K b = 1.8 x 10 -5 0.10 - x x x K b = ] NH [ ] OH ][ NH [ 3 4 - + = 1.8 x 10 -5 x 10 . 0 x 2 - = K b = 1.8 x 10 -5 Solving for x, making the usual approximation that x is small compared with 0.10, we get: x = [OH - ] = 1.34 x 10 -3 M pOH = -log[OH - ] = - log (1.34 x 10 -3 ) = 2.87 pH = 14.00 - pOH = 14.00 - 2.87 = 11.13 Percent ionization = 10 . 0 10 x 34 . 1 3 - x 100 = 1.34% Besides NH 3 , most other bases are organic molecules known as amines, which are close relatives of NH 3 . One or more carbon atoms replace the hydrogen atoms. Amines are weak bases, since the N atom still has a lone pair of electrons, which is a location which attracts an H + ion. The simplest amine is methylamine,
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lecture+10sf-1 - pH of Strong Bases The most common...

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