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9 December, 18:40

Which of the following acids should be used to prepare a buffer with a pH of 4.5? A. HOC6H4OCOOH, Ka = 1.0 x 10^-3B. C6H4 (COOH) 2, Ka = 2.9 x 10^-4C. CH3COOH, Ka = 1.8 x 10^-5D. C5H5COOH, Ka = 4.0 x 10^-6E. HBrO, Ka = 2.3 x 10^-9

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  1. 9 December, 19:20
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    C. CH3COOH, Ka = 1.8 E-5

    Explanation:

    analyzing the pKa of the given acids:

    ∴ pKa = - Log Ka

    A. pKa = - Log (1.0 E-3) = 3

    B. pKa = - Log (2.9 E-4) = 3.54

    C. pKa = - Log (1.8 E-5) = 4.745

    D. pKa = - Log (4.0 E-6) = 5.397

    E. pKa = - Log (2.3 E-9) = 8.638

    We choose the (C) acid since its pKa close to the expected pH.

    ⇒ For a buffer solution formed from an acid and its respective salt, we have the equation Henderson-Hausselbach (H-H):

    pH = pKa + Log ([CH3COO-]/[CH3COOH])

    ∴ pH = 4.5

    ∴ pKa = 4.745

    ⇒ 4.5 = 4.745 + Log ([CH3COO-]/[CH3COOH])

    ⇒ - 0.245 = Log ([CH3COO-]/[CH3COOH])

    ⇒ 0.5692 = [CH3COO-]/[CH3COOH]

    ∴ Ka = 1.8 E-5 = ([H3O+].[CH3COO-]) / [CH3COOH]

    ⇒ 1.8 E-5 = [H3O+] (0.5692)

    ⇒ [H3O+] = 3.1623 E-5 M

    ⇒ pH = - Log (3.1623 E-5) = 4.5
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