Chapter 15

# Chapter 15 - COMMON-ION EFFECT Adding or subtracting the...

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COMMON-ION EFFECT Adding or subtracting the conjugate base (acid) of an acid (base) affects the pH of a solution by consideration of LeChâtelier’s principle. - This is nothing new. In other words, adding a common ion to solution decreases dissociation of acid or base. Example: A 1.0 M solution of HF as a pH of 1.58 K a (HF) = 6.8 x 10 -4 . How does the pH of the solution change when 42 g of NaF is added to the solution? Assume the volume remains constant at 2.0 L. First consider what happens to equilibrium when F - is added? HF (aq) D H + (aq) + F - (aq) [ ][ ] [ ] K H F HF a = + - - Equilibrium will shift to the left. - H + will decrease. - Since F - is a base, it is sensible that adding it will decrease H + (increase pH). Calculate initial concentration of F - .

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Example: 34.6 g of NH 4 Cl is added to 3.98 L of 0.0145 M solution of NH 3 . K b (NH 3 ) = 1.8 x 10 -5 . a) What is the pH of the original solution before the addition of the NH 4 Cl? NH 3 (aq) + H 2 O (l) D NH 4 + (aq) + OH - (aq) [ ] [ ] [ ] K NH OH NH b = + - 4 3 [NH 4 + ] [OH - ] [NH 3 ] Initial 0 0 0.0145 M Change x x – x Equil. x x 0.0145 – x ( 29 K x x b = - = × - 2 5 00145 18 10 . . Assume that x is small. x 2 5 00145 18 10 . . = × - ( 29 x 2 5 7 00145 18 10 2 6 10 = × = × - - . . . x = 5.7 x 10 -4 pOH = 3.29 pH = 10.71 b) Calculate the pH of the solution after the addition of the NH 4 Cl. Assume volume remains constant. Calculate initial concentration of NH 4 + . [NH 4 + ] [H + ] [NH 3 ] Initial 0.162 0 0.0145 M Change – x x + x Equil. 0.162 – x x 0.0145 + x ( 29 K x x x a = + - = × - 0 0145 0162 56 10 10 . ( . ) .
Assume that x is small. ( 29 x 0 0145 0162 56 10 10 . ( . ) . = × - ( 29 x = × = × - - 0162 56 10 00145 6 26 10 10 9 . . . . pH = 8.20 BUFFERS - Buffers are solutions of conjugate acid/base pairs. - Buffers change pH slowly when acid or base is added. - Buffers take advantage of common-ion effect Consider solution of 2.0 M HC 2 H 3 O 2 and 1.0 M NaC 2 H 3 O 2 . HC 2 H 3 O 2 (aq) D H + (aq) + C 2 H 3 O 2 - (aq) - Because of common-ion effect, acid will dissociate very little. When a strong acid is added to the solution, it neutralizes the base, C 2 H 3 O 2 - . HCl (aq) + C 2 H 3 O 2 - (aq) HC 2 H 3 O 2 (aq) + Cl - (aq) When a strong base is added to the solution, it neutralizes the acid, HC 2 H 3 O 2 . NaOH (aq) + HC 2 H 3 O 2 (aq) NaC 2 H 3 O 2 (aq) + H 2 O (l) What happens to the solution if we add enough HCl to change to acetate concentration from 1.0 to 0.9? - Base is converted to acid. Therefore, acetic acid concentration must change from 2.0 to 2.1. HCl (aq) + C 2 H 3 O 2 - (aq) HC 2 H 3 O 2 (aq) + Cl - (aq) 0.1 M 1.0 M 2.0 M 0.0 M 0.0 M 0.9 M 2.1 M 0.1 M before neutralization after neutralization

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- Now calculate pH ( 29 x( . ) . . 0 9
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Chapter 15 - COMMON-ION EFFECT Adding or subtracting the...

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