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    (Original post by Pomum)
    Hi,

    So the question I need help on is the following: "Calculate the hydrogen ion concentration and the pH of a 0.010 mol dm3 methanoic acid solution. State one assumption made in arriving at your answer."

    I've got the pH (it's 2.88) but I don't really understand the assumptions part even when I look at the mark scheme. I know it's a weak acid so it will only partially dissociate into its ions in aqueous solution. The mark scheme says: "assume x << 0.010 / 25 °C / negligible dissociation". Could someone explain this to me?

    Thanks,

    Pomum

    P.S. x is just the letter used to represent [H+]
    Yes, you assume that the concentration of the original acid does not change, i.e. there is no dissociation...

    This allows you to use the value for the acid concentration in the ka equation.

    The approximation is a good one, giving pH agreement with more complex methods of working out to 3 decimal places.
 
 
 
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