Sunday
November 23, 2014

Homework Help: science (chem)

Posted by christina on Wednesday, June 27, 2007 at 6:17pm.

In general when instructions are to let a solution cool to room temp would it be incorrect to put the bottle containing the solution into a cold water bath so it could cool faster? Would it affect the solution in an adverse way? or does it depend on what you are cooling?

Cooling under water would be ok. A cold water bath (such as shaved ice or ice water) could get the lower region of the flask too cold and some solid might precipitate. Of course some solutions don't have a solid as a solute; therefore, it does depend to a large extent upon what is being cooled. With regard to NaOH solution, I might let the solution cool under running water but I wouldn't put the flask in ice water. I don't have any experience with that and I suspect that solutions of about 0.1 M NaOH would not ppt any NaOH. However, I still wouldn't do it just on general prinicples.

I was referring to just plain tap water in a beaker and just floating the flask in there.
When I made the NaOH solution, I boiled the water again and left it in the bottle with the CO2 absorber and put that into a cold water bath, then the problems started….First I didn’t have enough boiled water and I found that out when I put the water + 5ml 50% NaOH so I left It in the volumetric 1000ml flask for ½ hr while boiling some more water, cooling it in bottle w CO2 absorber. Then adding it too the volumetric flask before adding it back to the bottle + CO2 absorber. Then comes titration problems today…I ATTEMPT to use the bottle to get NaOH solution by squeezing the bottle and end up getting NaOH solution going into the CO2 absorber as well as into the buret…not good at all…in the end finally I get it to go only into the buret after pouring it into the buret with the funnel first, but everytime I finish squeezing some NaOH solution into the buret air goes back up that glass rod and into the storage bottle which if I’m not wrong, would expose the solution to all the CO2 in the air or is that supposed to happen?
Anyways with the standardizing of the HCl I put the solution of the HCl+ Na2CO3 + M.O indicator on a hot plate to boil after titrating it and was sitting there for about 10 min (time a wasting) so I asked the tech if I could place it into a cold water bath, tech says fine, I do that but it turned orange again. I wasn’t sure if that was supposed to happen so I ask tech and get a shoulder shrug =(….is this what techs are for!? Is that supposed to happen?
I ask about color and get another “um” and was also told that I could calculate about how much HCl I should add from the eqzn [Na2CO3+2HCl=>H2CO3 + 2NaCl] but I’m not exactly sure how to do that

That was the reason I asked about the cold water bath since I'm kinda behind a few labs and the next one is Determination of Magnesium by complex formation titration in which I have to dissolve EDTA in water and wait for it too cool to room temp and since I'm behind I was thinking of a faster way to cool solutions which have just been heated but I'm not sure If for example I would have ppt come out of solution or some other unfunny rxn happen but I REALLY have got to speed up my pace in lab and all these waiting for solutions to cool to room temp on their own aren't helping...

For the 2nd part of the Determination of Mg by complex-titration, Determination of MgO by using EDTA as a titrant I have to use Eriochrome black indicator as a indicator, I was reading that the endpoint is when it turns blue but also that it also becomes pink after becoming blue.
When purifying the EDTA the crystal are supposed to be washed in a certain order with 1. Ethyl alcohol
2. acetone
3. ether
But why is that? And how many times with how much of each do you wash them with because it isn’t mentioned.

And I might as well ask about the lab after that while I’m at it… In my lab 7 (Determination of hardness of water) it was changed from CaCO3 to (CH3COO)2CaH2O
instead, would that change anything about the lab besides the molar mass in the calculation? In this lab I have to boil HCl to eliminate CO2 and I have the same Q I had last time on what’s the point if I have to let it cool to room temp (and if I can cool it with a cold water bath) and it will absorb CO2 again? After that I have to use Methyl red indicator and it’s a Q of the color again…Then I use 2ml buffer (ph10) and then titrate with a standard solution of EDTA + eriochrome black T. (what is eriochrome black T? I know from the lab before this one that it’s an indicator but what’s with the T?)

thanks DrBob
and I hope I’m not getting annoying…=)

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