Yttrium2
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Titration problems
Two questions:
Any advice on easy ways to remember the titration equation? Isn't there a formula, which one seems to work best?
When we did a titration in class, the endpoint of the titration was not neutral. How come we did not stop adding acid at neutral? Wouldn't neutral be
when all of the base is neutralized? I thought titrations stopped at neutral, previously.
Thanx
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blogfast25
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Quote: Originally posted by Yttrium2  | Two questions:
Any advice on easy ways to remember the titration equation? Isn't there a formula, which one seems to work best?
When we did a titration in class, the endpoint of the titration was not neutral. How come we did not stop adding acid at neutral? Wouldn't neutral be
when all of the base is neutralized? I thought titrations stopped at neutral, previously.
Thanx |
Titrations only have a neutral end-point if you titrate a strong base with a strong acid (or vice versa).
If you titrate a weak base (e.g. ammonia or an amine) with a strong acid the end-point is at pH slightly lower than 7.
If you titrate a weak acid (e.g. acetic acid) with a strong base the end-point is at pH slightly higher than 7.
The titration protocol will usually tell you what indicator to use.
Titration formula:
V1 = volume of titrant used, C1 = concentration of titrant used.
V2 = volume of analyte used, C2 = concentration of analyte used.
Formula: C1V1 = C2V2
Usually C2 is the unknown, so C2 = C1 x (V1/V2)
[Edited on 2-7-2015 by blogfast25]
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DraconicAcid
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Titrations stop when you've added exactly enough A to react with all of the B in your flask. It's a stoichiometry problem. There are formulas that
some students use, but I loathe them, because they get mixed up with the dilution formula, and shows that they have no idea what's really happen.
I tell my students that any stoichiometry problem can be solved with the same three basic steps, just like Dora the Explorer. Her map always tells
her to go somewhere in three steps (cross the bridge, go through the forest, and that's how you get to Grandma's house. You tell Dora, bridge,
forest, Grandma's house!). In stoichiometry, the three steps are a) convert to moles, b) apply the mole-to-mole ratio, and c) get whatever it is you
were asked to find (mass, concentration, percentage, etc). Moles, ratio, Grandma's house.
So if I have the volume and concentration of the base I used to titrate a known volume of a diprotic acid, I would a) convert the volume of the base
into moles (n = CV), b) use the mole-to-mole ratio from the balanced equation to determine the number of moles of acid (1:2 ratio), and c) divide the
number of moles of acid by the volume it was originally in, to get the original concentration.
If you do an acid-base titration of a strong acid with a strong base, the equivalence point will be neutral (when you add exactly enough acid to
neutralize all the base, there's nothing left, so it's neutral). The endpoint is when the indicator changes colour, and if the indicator doesn't
change colour until a pH of 9, the solution will be basic at the endpoint (which will be a fraction of a drop of titrant past the endpoint, so the
difference doesn't matter). If either the acid or the base isn't strong, then the equivalence point pH will not be 7. A weak acid titrated with a
strong base will give a solution of the conjugate base of the weak acid, which will be a weak base, so it will be basic.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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DraconicAcid
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| Quote: Originally posted by blogfast25 |
Titration formula:
V1 = volume of titrant used, C1 = concentration of titrant used.
V2 = volume of analyte used, C2 = concentration of analyte used.
Formula: C1V1 = C2V2
Usually C2 is the unknown, so C2 = C1 x (V1/V2) |
That's exactly what I hate to see my students using, because it only works in a 1:1 stoichiometry, and I also can't tell if they know what's going on,
or if they are using the dilution formula just because it's the only one they remember.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Volanschemia
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I much prefer the formula:
Mcompound to be titrated = Vtitrant / Vcompound to be
titrated * Mtitrant * Ccompound to be titrated / Ctitrant
Where C = Coefficient, V = Volume, M = Molarity.
[Edited on 3-7-2015 by TheAustralianScientist]
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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blogfast25
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| Quote: Originally posted by TheAustralianScientist | I much prefer the formula:
Mcompound to be titrated = Vtitratee / Vcompound to be
titrated * Mtitratee * Ccompound to be titrated / Ctitratee
Where C = Coefficient, V = Volume, M = Molarity.
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See, I wouldn't remember that in an exam. But then I wouldn't have to either! 
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blogfast25
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| Quote: Originally posted by DraconicAcid |
That's exactly what I hate to see my students using, because it only works in a 1:1 stoichiometry, and I also can't tell if they know what's going on,
or if they are using the dilution formula just because it's the only one they remember. |
Yeah, whatever. In case you hadn't noticed, I'm not one of your students!  Couldn't blame you if you were glad about that too...
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