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15 June, 00:34

Calculate the ph of a 0.200 m nach3co2 solution. ka for acetic acid, ch3co2h, is 1.8 * 10-5.

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  1. 15 June, 00:40
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    Annswer : The correct answer is : pH = 9.03

    To calculating pH of NaCH₃CO₂, we need to do following steps:

    Step 1 : Find kb for NaCH₃CO₂

    Since NaCH₃CO₂ and CH₃CO₂H are conjugates base - acid pair, we can calculate kb for NaCH₃CO₂ from ka of CH₃CO₂H using relation:

    Kw = ka x kb,

    Where Kw = ionic product of water = 1 x 10 ⁻¹⁴

    ka = dissociation constant of acid = 1.8 x 10⁻⁵

    kb = dissociation constant of base

    Plugging value of kw and ka in formula:

    1 x 10 ⁻¹⁴ = 1.8 x 10⁻⁵ x kb

    Dividing both side by 1.8 x 10⁻⁵

    1 x 10⁻¹⁴ / 1.8 x 10⁻⁵ = 1.8 x 10⁻⁵ / 1.8 x 10⁻⁵ x kb

    kb = 5.6 x 10 ⁻¹⁰

    Step 2 : Setting ICE (Initial, change and equilibrium concentration) Table.

    NaCH₃CO₂is strong base, so its ion CH₃CO₂⁻ undergoes hydrolysis and gives OH⁻ as follows:

    CH₃CO₂⁻ + H₂O → CH₃CO₂H + OH⁻.

    Initial (M) 0.200 - 0 0

    Change (M) - x - + x + x

    Equilibrium (M) 0.200 - x x x

    Step 3: Writing kb for reaction

    kb = [CH₃CO₂H] [OH⁻] / [CH₃CO₂⁻ ]

    Plugging equilibrium concentrations in above equation

    5.6 x 10 ⁻¹⁰ = [ x] [x] / [0.200 - x ]

    Neglecting x from 0.200 - x

    5.6 x 10 ⁻¹⁰ = [ x²] / [0.200 ]

    Multiplying both side by 0.200

    5.6 x 10 ⁻¹⁰ x 0.200 = [ x²] [0.200 ] / [0.200 ]

    x² = 1.12 x 10⁻10

    x = 1.06 x 10⁻⁵

    [CH₃CO₂H] = [OH⁻] = x = 1.06 x 10⁻⁵

    Step 4 : Calculating pOH

    pOH = - log [OH-]

    pOH = - log [ 1.06 x 10⁻⁵ ]

    pOH = 4.97

    Step 5: Calculating pH

    Relation between pH and pOH : pH + pOH = 14

    pH = 14 - pOH = 14 - 4.97

    pH = 9.03
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