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Acids_and_Bases

Acids_and_Bases - Acids and Bases Calculations for a...

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Acids and Bases Calculations for a Titration Plot HCl ! H + + Cl - HOAc ! H + + OAc - H 2 O ! " + + #" $ K w = [H + ][OH - ] K a and K w are the molar equilibrium constants Some basics from General Chem Ionization of Water [H + ][OH - ] = 1.0 x 10 -14 = K w at 25 C In pure water [H + ]=[OH - ] = 1.0 x 10 -7 M
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Calculate [OH - ] of acid in water 1 - x 10 -3 M solution of HCl. [OH - ]? HCl completely ionized H + is 1.0 x 10 -3 1.0 x 10 -3 [OH-] = 1.0 x 10 -14 [OH-] = 1.0 x 10 -11 M Calculate pH and pOH of 1.0 x 10 -7 M HCl in water solve quadratically x = 6.2 x 10 -8 total H + = 1.00 x 10 -7 + 6.2 x 10 -8 = 1.62 x 10 -7 pH = 6.79 pOH = 14.00 - 6.79 = 7.21 but CO 2 acidic in water Weak Acids and Weak Bases HOAc ! H + + OAc - (C - x) x x Calculate pH and POH 1.00 x 10 -3 M acetic acid from K a expression: (x)(x)/ (1.00 x 10 -3 - x) = 1.75 x 10 -5 = (K a ) pH = - lg 1.32 x 10 -4 = 3.88 pOH = 14.00 - 3.88 = 10.12
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K b example, Calc pOH, pH of 1.0 x 10 -3 M soln of NH 3 K b = 1.75 x 10 -5 = [NH 4 + ][OH - ]/[NH 3 ] NH 3 + H 2 O % &" 4 + + #" $ (1.00 x 10 -3 - x) x x (x)(x)/1.00 x 10 -3 = 1.75 x 10 -5 (x neglected because small relative to 10 -3 ) x = 1.32 x 10 -4 = [OH - ] pOH = - lg 1.32 x 10 -4 = 3.88 pH = 14.00 - 3.88 = 10.12 some salts when dissolved react with water salts of strong acids and strong bases are completely ionized; don’t react to give H + or OH - e.g. NaCl but salt of weak acid reacts with water: OAc - + H 2 0 ! HOAc + OH - salt of weak acid undissociated weak acid pH of salts of weak acids/bases Salts of Weak Acids/Bases sodium acetate is a salt of the weak acetic acid, HOAc: HOAc ! H + + OAc - i.e. sodium acetate is a weak base OAc - + H 2 0 ! HOAc + OH - ionization constant is K b reaction is called hydrolysis of salt ion
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Calc. of K b of salt of weak acid K b can be calculated from K a of acetic acid and K w multiply top and bottom by [H+] K a K b = K w inside the dashed line is K w , the rest is 1/K a : small K b indicates acetate is a weak base product of K a of a weak acid and K b of its conjugate base is always = K w note “conjugate” terminology (weak base formed from weak acid) Calc. of pH of salt of weak acid calc. same as any weak base approximation: x neglected if C A- is > 100 K b best to do quadratic - trivial for calculator and or compare with NH 3 (weak base) example - identical
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Salts of Weak Bases Similar equations can be written for salts of weak bases which also ionize in water BH + + H 2 O ! B + + H 3 O + Homework Study these expressions in Harris, e.g. 187 Work on the examples in Harris Summary - Salts Weak Acids/Bases Salts of weak acids or weak bases change pH of water. Salts of strong acids and bases don’t e.g. Na + Cl - + H 2 O ! (H + + OH - ) + Cl - + Na + HCl completely dissociated so no hydrolysis - solution pH determined by [HCl] Na + OAc - + H 2 O ! HOAc + OH - + Na + OAc - - salt of weak acid, Na + - salt of strong base HOAc not completely dissociated, some OH- will form - solution basic Summary (cont.) NH 4 + (Cl - ) + H 2 O ! NH 4 OH + H + NH 4 +
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