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1 December, 12:26

Calculate the concentration of acetic acid (pka 54.76) and sodium acetate necessary to prepare a 0.2 m buffer solution at pH 5.0

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  1. 1 December, 15:52
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    See explanation below

    Explanation:

    To solve this problem, we need to use the Henderson - Hasselbach equation which is the following:

    pH = pKa + log ([A⁻] / [HA]) (1)

    Where:

    [A⁻] = concentration of the acetate

    [HA] = concentration of the acetic acid.

    With the above expression, we can calculate the ratio of [A⁻] / [HA]. This concentration ratio will be used to calculate the individual concentrations. We know that a buffer is made as a mix of the acid and it's ion, in other words:

    [HA] + [A⁻] = [Buffer] (2)

    So, with the concentration ratio, we can replace it here in (2) and then, we can calculate each concentration required.

    First, let's use (1) to get the concentration ratio:

    5 = 4.76 + log ([A⁻] / [HA])

    5 - 4.76 = log ([A⁻] / [HA])

    0.24 = log ([A⁻] / [HA])

    [A⁻] / [HA] = 10⁰°²⁴

    [A⁻] / [HA] = 1.7378

    From this ratio, we can express either HA or A in function of the result. In this case, we will solve for A:

    [A⁻] = 1.7378[HA]

    With this expression, we can replace it in (2) to solve for [HA]:

    [HA] + 1.7378[HA] = 0.2

    2.7378[HA] = 0.2

    [HA] = 0.0731 M

    Now all we have to do is replace this value in [A] to get the concentration:

    [A⁻] = 1.7378 * 0.0731

    [A⁻] = 0.1270 M
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