Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: glucose8999 on August 19, 2005, 06:34:46 PM
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How would you separate a mixture of salt and sugar? Both must remain preserved; we need to find percent composition by mass.
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I already thought of burning the sugar and making it black, then removing it.. but wouldn't that mess up the mass as well as the chemical composition?
Another idea I had was to try to dissolve them in ethanol (a non-polar solvent) and see if one would dissolve and one wouldn't. Would salt dissolve in ethanol? Would sugar?
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This gives a good method for it: http://www.newton.dep.anl.gov/askasci/chem00/chem00719.htm (http://www.newton.dep.anl.gov/askasci/chem00/chem00719.htm)
Just a couple notes:
1. This DOES preserve both compounds, whereas burning one or the other does not. Burning sugar changes the chemical composition so drastically that you will NEVER be able to reconstitute it chemically.
2. If somebody could verify this for me, that'd be great...but I think that if you dissolve the sugar in acetone and water as it suggests, then all you'd have to do is leave the solution out for a while and let each of those liquids evaporate...unless sugar makes some kind of hydrated or acetonated complex...but I think that each of those solvents would evaporate totally and you could probably even eat the sugar after its all gone...Probably not commended, but is it possible?
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If you do not intend to seperate the mixture, but still obtain mass composition, I have an alternate method.
assumption: salt is NaCl
1. dissolve the mixture in water
2. add excess aqueous silver nitrate to the solution
3. filter the solution to obtain silver chloride
4. measure the mass of the silver chloride
from the mass of silver chloride, you can deduce the mass of NaCl in the mixture.
eg. u dissolved 100g of the mixture in 500ml of water, and you obtain 50g of AgCl.
number of moles of AgCl = 50/(107.9+35.5) = 0.349moles
NaCl(aq) + AgNO3 (aq) -> AgCl(s) + NaNO3 (aq)
from the stoichiometric ratio of the above reaction,
number of moles of NaCl in mixture = number of moles of AgCl formed = 0.349moles
mass of NaCl in mixture = 0.349 x molar mass of NaCl = 20.4g
mass of sugar in mixture = 100 - 20.4 = 79.6g
percentage mass composition = 20.4% salt and 79.6% sugar
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Dear All;
This Tread may be very old, but during the last time it is very often visited.
That’s why I think we should improve adequateness of its answers!
The “magic word” as sugar and salt must remain preserved is: “Fractionated Crystallisation”,
because solubility of Salt: at 20°C 35.88g; at 100°C 39.2g per 100ml of Water;
and the solubility of Sugar: at 20°C 200g; at 100°C ~520g per 100ml of Water.
For more explations you may visit:
http://www.sci-journal.org/index.php?template_type=report&id=23&htm=reports/vol3no2/v3n2a5.html&link=reports/home.php&c_check=1
or read the document in the Attachment.
I hope this gives you a better understanding.
Good Luck!
ARGOS++