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29 December, 00:47

What mass of sodium benzoate should be added to 150.0 ml of a 0.15 m benzoic acid solution in order to obtain a buffer with a ph of 4.25? (assume no volume change.) ?

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  1. 29 December, 04:25
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    When we know the Pka value = 4.19

    so, we are going to use H-H equation:

    PH = Pka + ㏒[benzoate/benzoic acid]

    when we have PH = 4.25

    and Pka = 4.19

    so by substitution:

    4.25 = 4.19 + ㏒[benzaoate / benzoic acid]

    ∴ [benzaoate/benzoic acid] = 1.148 M

    when the [benzoic acid ] = 0.15 m

    ∴ [benzaoate] = 1.148M * 0.15m

    = 0.1722 M

    ∴ moles of sodium benzoate = molarity * volume L

    = 0.1722 M * 0.15 L

    = 0.02583 moles

    ∴ the mass = moles * molar mass

    = 0.02583 moles * 144 g/mol

    = 3.72 g
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