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equivalence point - definition of an equivalent

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umdnjmed

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According to EK Chemistry, an equivalent is the mass of an acid or base necessary to produce or consume a mole of protons.

Does this mean that one equivalent of NaOH is 1mole, while one equivalent of H2SO4 is 0.5mol?

If the latter is true, in a titration of both, the equivalence point (which is the point at which there are equal equivalents of acid and base) should contain 1mole of NaOH and 0.5moles of H2SO4. Is it wrong therefore to say that at the equiv point,
moles acid = moles of base?

Thanks in advance.
 

milski

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You're mostly on the right track but you're mixing too many things together. Equivalence points relate to one compound - either a base or an acid. You either titrate acid with a base with known molarity or vice versa.

NaOH will have a single equivalence point and yes, for 1 mole of it, it will be 1 mole of acid. H2SO4 will have two equivalence points - one for each proton being removed. The second equivalence point will be twice the first, for obvious reasons.
 

umdnjmed

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It makes sense that at the equivalence point for in a NaOH titration curve, one mole of an acid like HCl would be used to neutralize the base. However, if the acid being used is H2S04 is used as the titrant, complete dissociation of 1mole of the acid will result in 2 moles of H+.

I guess the question i'm asking is, at the equivalence point for NaOH titrated with H2SO4, in order for complete neutralization to occur, will you have equal moles of both? or will there be half as many moles of H2SO4, since it is diprotic.
 

rjosh33

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I guess the question i'm asking is, at the equivalence point for NaOH titrated with H2SO4, in order for complete neutralization to occur, will you have equal moles of both? or will there be half as many moles of H2SO4, since it is diprotic.

Yes, you would have half as many moles of sulfuric acid as sodium hydroxide. However, there is no contradiction with the neutralization definition that moles acid = moles base. Like you and milski noted earlier, there are two acidic protons to sulfuric acid, and they are said to dissociate completely in the presence of a base like OH-. Therefore, 1 mole H2SO4 goes to 2 moles H+, which is the functionally acidic atom in question. So, to neutralize 1 mole NaOH, we need 1 mole H+, which is produced by 0.5 moles H2SO4.

Note -- the annoying stoichiometric conversions are why you generally use HCl as the strong acid in titrations rather than a diprotic like sulfuric acid.
 

umdnjmed

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I see, so moles of acid and base refers to the moles of H+ and OH- respectively. Thank you.
 
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