1,000,000 Question Thread :D

[kenhung123]

Thank you!

[Greggler]

Hey guys, just thought i'd post a query i have.

In redox titrations reactants include things such as KMnO4.
However when wiriting/balancing out these complex equations with the oxygens and the water etc. it is simply written down as an ion (MnO4-)
The K simply disapears from the equation (i presume as a spectator ion?)
Why is this. I guess that this is because of something to do with solubility rules etc. But then why does this never happen in acid/base titrations? (or does it?)

Any info that could help clear this up would be greatly asppreciated

[/0]

Hmm, I think in solution, and ions are actually dissociated. We only write it as for brevity, but we could in fact write it as . When is a spectator ion it cancels from both sides.

[Greggler]

Yeah thanks for clearing that up /0. I figured they had dissociated or something. Just wanted to be 100% sure.

By the way, is this just a thing with titrations?

[longy1991]

spectator ions apply to pretty much everything yeah.

[kenhung123]

Why do you need to rinse the pipette with the solution used but not the conical flask when in volumetric analysis?

[chem-nerd]

because the pipette is required to deliver an accurate volume of a specific CONCENTRATION whereas the conical flask needs an accurate NUMBER OF MOLE.

If you rinse the conical flask with the solution you are adding additional and unknown amount of mole rather than just the accurate amount delivered by the pipette.

If you rinse the pipette with water it will no longer deliver an accurate volume of KNOWN concentration.

[kenhung123]

Will we be asked such a question?

[chem-nerd]

yeah, it's not uncommon to come across this in an exam/SAC. What should a particular piece of glassware be rinsed with and what effect would it have on the final result if it was rinsed incorrectly?

[kenhung123]

It should be fairly straight foreward if you follow the calculations right? E.g. rinse the pipette with water=more water less mole so less mole in the burette will react with the aliquot? Or does it require lots of imagination =.=

[kenhung123]

My observation: If you add water into pipette, you will have the same volume but less mole and concentration.
If you titrate this with a solution in burette, that means there will be less requirement because less is needed to react. Therefore the calculated mole with n=CV is accurate.

[Potter]

My observation: If you add water into pipette, you will have the same volume but less mole and concentration.

I'm confused if you're talking about a Burette or a pipette..
Also, If you add water into the thing the volume and concentration will change. The Mol will be constant.

[kenhung123]

If there is some water in the pipette then the volume is already increased and less number of moles can enter right? Also  the concentration much change.

[natty]

If there is some water in the pipette then the volume is already increased and less number of moles can enter right? Also  the concentration much change.

Adding water= decreases concentration, but same number of mole.
If there is water in the pipette before adding the substance, less moles will be delivered to the flask as less substance will be added. The concentration may not change hugely but it can affect the calculations.

[kenhung123]

Thanks!

Why does the pH change rapidly when the equivalence point is reached?