Titration things... (1 Viewer)

[Aerlinn]

I was thinking... what are the safety precautions of titrations, (apart from wearing lab coats and safety glasses)?
I was doing a question where we had to write out the ionic equation of this reaction:
CH3COOH(aq) + KOH(aq) ---> CH3COOK(aq) + 2H20(l)
When I separated the ions and cancelled them on either side, I got
H+(aq) + OH-(aq) ---> H2O(l)
But that's not the answer... instead it's CH3COOH(aq) + OH-(aq) --->CH3COO-(aq)+ H20(l)
Don't understand it...
Quick query about pipettes, because we've been asked to draw them, I think they're designed to allow for a little bit of solution at the bottom? So, after delivering your 20ml of solution, the meniscus would be near the bottom of the pipette since there'd be some solution left, if you were to draw it, yes?

I've noticed that sometimes you get examples of titration that dilute the
sample before titration occurs... why is dilution done sometimes? Supposedly, if it were not done, you'd get much greater titre volumes, but that doesn't make too much sense to me... an explanation, perhaps?

:wave:

 

[xiao1985]

becareful of glasses (they can be broken, despite common believes), acids (burns), bases(burns more), don't eat in lab, don't drink in lab

2nd question: well, this is really grey i guess... in solution, you have both ch3coo - H+ and CH3COOH... so i say you ain't wrong either...

correct (well, you won't be able to see the meniscus, it's just a drop of water at the bottom of pipette

3 reasons:
a) if you are titrating something with color, the end point will be much easier to see once you diluted it (think fanta)

b) if you dilute the stuff in burette, you'd minimise your non-systematic error... for eg, when you read burette, you incur a +- 0.5mL error... if you do with conc 'd stuff in burette, say you used 10mL, the error would be 5%... whereas, if you dilute it, now you need 100mL, the error would be reduced to 0.5%... same goes with the sample in the conical flask... you will have reminants of sample left pipette, on beaker, etc etc... dilute minimize the error you get ...

c) more stuff = spend more money... and clearly if you are working with something worth 4000 bucks/100g, you dont' want to chuck 200g for titration...

 

[the-1-n-only-me]

word of caution about the pipettes from my chem teacher.

apparently, since we're chemists, we use muscles in our fingers, not in our arms to stick the lil suction ball things on top. my chem teacher either witnessed or heard of a uni student who used the muscles in his arms to stice the end of the pipette in the sucking thing. needless to say, something slipeed, and the pipette went through hand, and the blood quickly filled the pipette and started spurting out the end. again.. you have been cautioned

ps. i dunno how true this story is. but its an interesting touch to chemistry.

 

[kormy]

we did a titration experiment for an aessessment last week, and we had to use the same chemicals as what you mentioned, except our question asked something about vinegar. the question was really weird, like we had to say if home brand vinegar complies to some law that says it must have at least 40g/L acetic acid. So just be careful of obscure questions... we had to dilute the vinegar to a 10% solution, then titrate, then work out the question. it took like 3+ hrs to do...
Firstly, we had to dilute it as part of the method because if you dont dilute it, then too much KOH would have been needed to make the CH3COOH neutral, and the burette only has 50mLs of KOH. we all used around 19mLs, so if we didnt dilute it, then wewould have had to use like 190mLs (we had to make a 10%solution).
Secondly, your equation. the original equation is CH3COOH + KOH --> KCH3COO + H2O, or someting like that... Anyway, i think its the K+ ions that cancel out...

 

[Aerlinn]

Wonder why the CH3COO- ions don't? Anyways, thanks for everyone's insight.
Although I thought that because KOH was a strong base, if it went in the burette, it would be fine because it wouldnt need that much to neutralise the acid? Wouldn't it be the other way around, ie. putting the CH3COOH in the burette that would become a problem, since CH3COOH is weak, so, it would require heaps of CH3COOH to neutralise the KOH?

 

[airie]

Wonder why the CH3COO- ions don't? Anyways, thanks for everyone's insight.
Although I thought that because KOH was a strong base, if it went in the burette, it would be fine because it wouldnt need that much to neutralise the acid? Wouldn't it be the other way around, ie. putting the CH3COOH in the burette that would become a problem, since CH3COOH is weak, so, it would require heaps of CH3COOH to neutralise the KOH?

The reason you can't "cancel out" CH3COO- ions is cos most of them are not in ionised form, but in fact in acetic acid molecule form. So you'd need to show that it's not simply an interaction between H+ and OH- ions, but also the dynamic equilibrium between H+ and CH3COO- ions.

Not sure about the acid in burette/base in burette business, but in China there's one for acids and one for bases, so that solves the problem I guess these days you could put either in the burette, except my chem teacher said that in the "old days" you avoid putting acids in the burette due to fear of corrosion to the tube. And yeah, I do think the strength of the acid (and hence the volume required) becomes of an issue as well

 

[xiao1985]

the burette thing is because originally, bases strew up the plastic turning thing on the burette...