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Ok, I do OCR A Level Chemistry..

And I was wondering, as different books give different definitions..
I always thought the equivalence point was where "equal moles of OH- ions have been added as there are H+ present" (assuming the alkali is being added to the acid). Some books say it is when equal "volumes" of OH- ions have been added...so which is it?! It makes a difference..as shown below...

All the examples i've seen use monoprotic acids, for example:
HCl + NaOH -> NaCl + H20.
(where both the HCl and NaOH are of the same concentration-e.g. 0.1M).
So therefore, say 25cm cubed of acid is placed in a conical flask, what volume of NaOH will be required to make up the equivalence point? (as they sometimes ask you to sketch curves in the exam questions)

Moles of acid =25/1000 X 0.1 = 2.5 X 10-3.
From the molar ratio of the equation; 1 mole acid: 1 mole alkali, therefore 2.5 X 10-3 moles acid, therefore, volume of NaOH required = 25cm cubed.

But what about when the ratios in the equation are different? Does this make a difference to the equivalence point? e.g.
H2SO4 + 2NaOH -> Na2SO4 + 2H20.

Again, say both the acid and the alkali are of 0.1M concentration, what volume of alkali is required to make up the equivalence point?
Again, 25cm cubed of acid was placed in the conical flask, and the alkali added..
So, moles of acid: 25/1000 X 0.1 = 2.5 X 10-3.
IF equivalence is when equal MOLES of acid have been added as alkali present...
Then do the molar ratios in the equation play a part to the volume of alkali required to form the equivalence point?? e.g. is the equivalence point when 2.5X10-3 moles of acid have been added, or does it depend on the molar ratios in the equation?
E.G. 2 Moles NaOH : 1 Mole H2SO4, so is the equivalence point when (2 X 2.5 X 10-3) moles of alkali have been added???
Sorry about the long post, I think I have confused myself...possibly by over-revising this topic?! No books have made it particularly clear, and now I am starting to get into a bit of a panic....

Thank you so much in advance! I hope it makes sense...

[EierVonSatan]

Okay the equivalance point is not wehn the concentrations of ions are in the same amount, it is when the acid and base (for monoprotics anyway) are in equal amount i.e. for HCl + NaOH -> NaCl + H20 when the number of moles of HCl = number of moles of NaOH. The complication with diprotic acids such as H2SO4, is that they have two equivalance points as there are two different pKa values for each dissociation

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Originally Posted by EierVonSatan

Okay the equivalance point is not wehn the concentrations of ions are in the same amount, it is when the acid and base (for monoprotics anyway) are in equal amount i.e. for HCl + NaOH -> NaCl + H20 when the number of moles of HCl = number of moles of NaOH. The complication with diprotic acids such as H2SO4, is that they have two equivalance points as there are two different pKa values for each dissociation

Thank you!!!! Obviously I dont need to know about two equivalence points etc...it's not on my syllabus, so i'm just overcomplicating things!!
So thank you very much!!