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Ch17_Salts_5

# Ch17_Salts_5 - 1 2 3 Acid-Base Properties of Salts • MX...

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Unformatted text preview: 1 2 3 Acid-Base Properties of Salts • MX + H2O ----> acidic or basic solution? • Consider the CATION CATION Is it NH4+? Is Is it Al3+ or a transition metal cation? or cation? • Consider the ANION ANION Is it a weak acid (e.g., H 2PO4-)? Is it the conjugate base of a weak acid? • See Table 17.5, page 822—and note connection with Table 17.4. Acid-Base Properties of Salts MX + H2O ----> acidic or basic solution? Consider NH 4Cl NH4Cl(aq) ----> NH4+(aq) + Cl-(aq) Cl(aq) aq) aq) (a) Reaction of Cl- with H2O with Cl - + H2O ----> HCl + OHbase acid acid base base Cl- ion is a VERY weak base because its ion conjugate acid is strong. Therefore, Cl- ----> neutral solution ----> Acid-Base Properties of Salts NH4Cl(aq) ----> NH4+(aq) + Cl-(aq) Cl(aq) aq) aq) (b) Reaction of NH 4+ with H2O with NH 4+ + H2O ----> NH3 + H3 O+ acid base base acid acid NH4+ ion is a moderate acid because its ion conjugate base is weak. Therefore, NH4+ NH ----> acidic solution ----> Acid-Base Properties of Salts Calculate the pH of a 0.10 M solution of Na 2CO3. Na+ + H2O ---> neutral CO32- + H2O ¸ HCO3- + OHbase Step 1. acid acid base Kb = 2.1 x 10 -4 2.1 Set up concentration table [CO32-] [HCO3-] [OH-] 0 +x x 0 +x x 4 Acid-Base Properties of Salts Calculate the pH of a 0.10 M solution of Na CO3. 2 Na+ + H2O ---> neutral CO32- + H2O ¸ HCO3- + OHKb = 2.1 x 10-4 2.1 base acid acid base 5 Acid-Base Properties of Salts Calculate the pH of a 0.10 M solution of Na CO3. 2 Na+ + H2O ---> neutral CO32- + H2O ¸ HCO3- + OHKb = 2.1 x 10-4 2.1 base acid acid base 6 Step 2. Solve the equilibrium expression [HCO3 - ][OH- ] K b = 2 .1 x 10-4 = [CO 32 ] x2 0.10 - x Step 3. Calculate the pH [OH-] = 0.0046 M pOH = - log [OH-] = 2.34 pH + pOH = 14, so pH = 11.66 , and the solution is ________. pH initial 0.10 change -x equilib 0.10 - x Assume 0.10 - x 0.10, because 100•K b < Co x = [HCO3-] = [OH-] = 0.0046 M Page 1 ...
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