Saturday
August 23, 2014

Homework Help: Chemistry

Posted by Waleed on Sunday, June 11, 2006 at 1:51pm.

Hi, my question is:
"Classify each of the following as saturated, supersaturated, or unsaturated:

KNO3 90g/100g at 82*C
NaCl 36g/100g at 23*C
KCl 35g/100g at 18*C
NaNO3 90g/100g at 28*C

How do I determine if they are saturated, supersaturated, or unsaturated?


Since the question is so specific regarding temperature, you must have some data you have collected or that is a part of your work (such as an experiment). You don't mention the solvent? Is it water? And is the solvent the grams of solvent or grams of solution? For example, should the first one read 90 g KNO3 in 100 g water or 90 g KNO3 in 100g of solution? Look in your reference for the solubility of KNO3, for example. I have a reference at home that gives an approximate solubility of 36 g KNO3 in 100 mL water @ 25 degrees C, The value at 82 degrees C will be different but we would say the solution was supersaturated if the question was for 25 degrees C.
As an example, and using the solubility at 25 degrees C as 36 g KNO3/100 mL, a soln would be classified as follows:
1 g/100 mL would be unsaturated becasue it COULD dissolve more.

36 g/100 mL would be saturated since that is the limit of its solubility. Another crystal added to that solution would simply float to the bottom of the flask and not dissolve.

If by some hook or crook we dissolved 50 g KNO3/100 mL water at 25 degrees C., we would call it supersaturated because the solvent has dissolved MORE THAN what is required for a saturated solution; thus the name supersaturated. (You may wonder how one does that. For some salts we could raise the temperature to near boiling, dissolve more than would be required for a saturated solution at 25 C., then let the solution cool slowly and without agitation. Some salts will not precipitate the excess amount of salt; thus we have a supersaturated solution. I hope this helps.

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