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Acid and bases calculations: assumptions? Watch
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Last edited by Pomum; 27-07-2013 at 10:40.
- 11-04-2013 17:31
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(Original post by Pomum)
- 11-04-2013 17:40
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?
P.S. x is just the letter used to represent [H+]
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.