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pH calculations (simple)

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TSR's new app is coming! Sign up here to try it first >> 17-10-2016
    • Thread Starter

    I've never really though about this until now.
    When a question says something like, a 30cm3 sample is diluted by 50cm3 water, find the pH of the solution formed.
    Should you take into account the moles of H+ that is in the water that is added (1) or would you just assume that the water doesn't dissociate at all (2). I have always assumed it is the latter but when thinking about it to get a more accurate answer you should consider the dissociation of water right? I know to 2dp it wouldn't usually make any difference but t would lose method marks right.
    Which one is correct and more importantly which one should I use in an exam?

    pH calcs will always be done at concs so low that you can forget about the ionisation of water.

    Use your latter thought, esp. in exams.
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