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13 January, 17:32

Caffeine (c8h10n4o2) is a weak base with a pkb of 10.4. part a calculate the ph of a solution containing a caffeine concentration of 430 mg/l.

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  1. 13 January, 20:49
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    Concentration of caffeine is 430 mg/L = 0.43g/L

    The molar mass of caffeine is 194.19 g/mol

    Therefore the molarity is:

    Molarity = (0.43/194.19) mol/L

    Molarity = 0.002214 mol/L

    Molarity = 0.002214 M

    Given pKb = 10.4:

    Kb = 10^-pKb = 10^ - 10.4 = 3.981 x 10^ - 11

    Kb is equivalent to:

    Kb = [caffeine H+][OH-] / [caffeine]

    3.981 x 10^ - 11 = [caffeine H+][OH-] / (0.002214)

    [caffeineH+][OH-] = 8.815 x 10^ - 14

    But since:

    [caffeineH+] = [OH-]

    Hence,

    [OH-]^2 = 8.815 x 10^ - 14

    [OH-] = 2.969 x 10^ - 7

    The formula for pH is:

    pH = 14 + log [OH-]

    pH = 7.47
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