Does the pH make sense Which should win the acidic propanoic acid or the basic

Does the ph make sense which should win the acidic

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Does the pH make sense? Which should win, the acidic propanoic acid or the basic sodium propanoate? HP and P - are in solution. The one that is stronger will govern the pH. K a =1.3x10 -5 ; K cb =7.7x10 -10 HP + H 2 O H 3 O + + P - K a =1.3x10 -5 P - + H 2 O OH - + HP K cb = 7.7x10 -10 In comparing the strength of HP as an acid, to P - as a base, since K a > K cb then HP is a stronger acid than P - is a base, and a pH <7 is reasonable. (If K a were 6.2x10 -10 as in HCN, and K cb were 1.3x10 -5 , as in CN - , then the final pH would have been > 7.)
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CHEM162-2012 CHAPTER 17 7 SEVEN STRONG ACIDS ET: Discuss strong acids and strong bases and the strengths of their conjugate bases and acids; practice both. Very weak base (i.e., not a base; K a Acid spectator ion)** ~10 9 HI + H 2 O H 3 O + + I - K cb = ~10 -23 ~10 8 HBr + H 2 O H 3 O + + Br - K cb = ~10 -22 ~10 6 HCl + H 2 O H 3 O + + Cl - K cb = ~10 -20 H 2 SO 4 + H 2 O H 3 O + + HSO 4 -*** K cb = ~0 HNO 3 + H 2 O H 3 O + + NO 3 - K cb = ~0 HClO 4 + H 2 O H 3 O + + ClO 4 - K cb = ~0 HClO 3 + H 2 O H 3 O + + ClO 3 - K cb = ~0 **These conjugate bases are known as “nominal bases”, i.e., a base by definition, but effectively not a base. ***Although HSO 4- is effectively not a base, it is effective as an acid. HSO 4 - + H 2 O H 3 O + + SO 4 2- All other acids are weak acids, e.g., HAc (acetic acid), H 2 SO 3 , RNH 3 + , BF 3 K a = 1.8x10 -5 HAc + H 2 O H 3 O + + Ac - K cb =5.6x10 -10 K a = 6.2x10 -10 HCN + H 2 O H 3 O + + CN - K cb =1.6x10 -5 Note that the stronger the acid, the weaker the CB; the weaker the acid, the stronger the CB. Arrhenius acid: Anything that provides a proton, e.g., HA, H 2 SO 3 , RNH 3 + LB Acid: Anything tending to give up a proton, e.g., HA, H 2 SO 3 , RNH 3 + Lewis Acid: Anything that accepts an electron pair, e.g., BF 3 STRONG BASES (Most Group 1A and 2A hydroxides [not HOH]) e.g. Very weak acid (i.e., not an acid; Base spectator ion) LiOH OH - + Li + NaOH OH - + Na + Ba(OH) 2 2OH - + Ba 2+ All other bases are weak bases, e.g., RNH 2 , CO 3 2- Arrhenius Base: Anything that provides an OH - group, e.g., NaOH LB Base: Anything that tends to react with a proton, e.g., OH - , RNH 2 , CO 3 2- Lewis Base: Anything that donates an electron pair, e.g., OH - , RNH 2
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CHEM162-2012 CHAPTER 17 8 ET: First discuss equations, then reverse equations invert K, then Ka x Kb = 10 -14 Write the equil. equations and identify K’s for the following reactions: Weak acid + water (e.g., acetic acid + H 2 O) : HA + H 2 O H 3 O + + A - 1 Assume K a = ~1 x 10 -5 Conjugate base + water (e.g., sodium acetate + H 2 O) : A - + H 2 O HA + OH - 3 K cb = K w /K a = (1 x 10 -14 )/(1 x 10 -5 ) = 1 x 10 -9 Conjugate base + H 3 O + (e.g., sodium acetate + H 3 O + ) :A - + H 3 O + HA + H 2 O 2 K = 1/K a = 1/(1 x 10 -5 ) = 1 x 10 5 (or bring to completion; then solve either using HH or reverse + K a ) Weak acid + strong base (e.g., acetic acid + sodium hydroxide) : HA + OH - H 2 O + A - 4 K = 1/(K w /K a ) = K a /K w = (1 x 10 -5 )/(1 x 10 -14 ) = 1 x 10 9 (or bring to completion; then solve either using HH or reverse + K a ) Weak base + water (e.g., NH 3 + H 2 O) : RNH 2 + H 2 O RNH 3 + + OH - Assume K b = ~1 x 10 -5 Conjugate acid + water (e.g., sodium acetate + H 2 O) : RNH 3 + + H 2 O RNH 2 + H 3 O + K ca = K w /K b = (1 x 10 -14 )/(1 x 10 -5 ) = (1 x 10 -9 ) Conjugate acid + OH - (e.g., ammonium chloride + OH - ) : RNH 3 + + OH - RNH 2 + H 2 O K = 1/K b = 1/(1 x 10 -5 ) = 1 x 10 5 (or bring to completion; then solve either using HH or reverse + K b ) Weak base + strong acid (e.g., NH
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