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23 January, 22:11

What is the ratio of the concentrations of H2PO4 and HPO4 2 at (a) pH 7.0; (b) pH 7.5; (c) pH 8.0?

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  1. 23 January, 22:23
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    A. 1.62

    B. 0.513

    C. 0.162.

    Explanation:

    pH = pKa + log [ HPO42-] / [H2PO4-]

    Assuming, pKa is 7.21

    Ka = 6.2 x 10^-8

    pKa = 7.21

    A.

    7.0 = 7.21 + log [HPO42-] / [HPO4-]

    - 0.21 = log [ HPO42-] / [H2PO4-]

    [ HPO42-] / [ H2PO4-] = 10^-0.21

    = 0.617

    [H2PO4-] / [HPO42-] = 1 / 0.617

    = 1.62

    B.

    7.5 = 7.21 + log [HPO42-] / [H2PO4-]

    0.29 = log [HPO42-] / [H2PO4-]

    [HPO42-] / [H2PO4-] = 10^0.29

    = 1.95

    [H2PO4-] / [HPO42-] = 1/1.95

    = 0.513

    8.0 = 7.21 + log [HPO42-] / [ H2PO4-]

    0.79 = log [HPO42-] / [H2PO4-]

    [HPO42-] / [H2PO4-] = 10^0.79

    = 6.17

    [H2PO4-] / [HPO42-] = 1/6.17

    = 0.162
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