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10 June, 18:27

Which of the following solutions would be best to buffer a solution near pH 5? (a) 1.0 x 10-5 M HCl (b) 1.0 x 10-5 M NaOH (c) A solution containing approximately equal concentrations of hypochlorous acid (HOCl, Ka = 2.9 x 10-8) and the dissolved salt of its conjugate base sodium hypochlorite (NaOCl). (d) A solution containing approximately equal concentrations of potassium hydrogen phthalate (C8H5KO4, Ka = 3.91 x 10-6) and the dissolved salt of its conjugate base potassium phthalate. (e) A solution containing approximately equal concentrations of ammonia (Kb = 1.8 x 10-5) and the dissolved salt of its conjugate acid ammonium chloride.

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  1. 10 June, 20:11
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    Option A: 1*10^-5 M HCl has a pH of 5

    Explanation:

    a) 1.0 * 10^-5 M HCl

    HCl = a strong acid

    pH of HCl = - log[H+]

    ph of a 10^-5 M HCl = - log (10^-5) = 5

    (b) 1.0 * 10^5 M NaOH

    pOH = - log (10^-5) = 5

    pH = 14 - 5 = 9

    (c) A solution containing approximately equal concentrations of hypochlorous acid (HOCl, Ka = 2.9 * 10^-8) and the dissolved salt of its conjugate base sodium hypochlorite (NaOCl).

    pH = pKa + log[NaOCl/HOCl]

    Since [NaOCl] / [HOCl] = 1

    pH = pKa + log (1)

    pH = pKa

    pH = - log (2.9 * 10^-8)

    pH = 7.54

    (d) A solution containing approximately equal concentrations of potassium hydrogen phthalate (C8H5KO4, Ka = 3.91 * 10^-6) and the dissolved salt of its conjugate base potassium phthalate.

    pH = pKa + log[potassium phthalate/C8H5KO4)

    Since [potassium phthalate]/[C8H5KO4] = 1

    pH = pKa

    pH = - log (3.91*10^-6)

    pH = 5.41

    e) A solution containing approximately equal concentrations of ammonia (Kb = 1.8 * 10^-5) and the dissolved salt of its conjugate acid ammonium chloride.

    pOH = pKb + log[NH4Cl]/[NH3]

    Since [NH4Cl]/[NH3] = 1

    pOH = pKb

    pOH = 4.74

    pH = 14-4.74 = 9.26
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