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OCR B Salter's Exam Question Help??

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Why bother with a post grad? Are they even worth it? Have your say! 26-10-2016
    • Thread Starter

    Not sure if this is the right place to post this so I'm sorry in advance!!
    Currently one day away from my F335 exam and I thought I was feeling confident until I came across a past maths-y question.

    I understand the method used but the rearrangement of the Ka = [H+] x [salt] / [acid] has really confused me!!


    The question is as follows:

    A student prepared 1dm3 of a pH 7.0 buffer solution based on the system:

    H2PO4- H+ + HPO42-

    The solution was prepared by dissolving Na2HPO4 and NaH2PO4 in deionised water. The concentration of Na2HPO4 in the prepared buffer solution was 0.10moldm-3.

    Calculate the mass of solid NaH2PO4 (Mr = 120) that the student used in preparing this buffer.

    Any help would be GREATLY appreciated!! Thank you
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