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Date Posted: 07:58:03 08/02/01 Thu
Author: shufei
Subject: Practice 2

Hi!
I have another question for practice 2 question 8. Why is the answer not D but B?

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[> Re: Practice 2 -- Mr. Yeo Woei Ter, 10:17:41 08/02/01 Thu

Hi Shufei!

First of all, you must know that a saturated solution is one which contains as much dissolved solute as possible at a particular temperature.

At 60 degree celcius, the solubility (as given in the question) of potassium nitrate is 106 g / 100 g of water. That means that the maximum mass of potassium nitrate that can dissolve in 100 g of water is only 106 g.
Therefore, if we have only 50 g of water at this temperature, then at most 106/2 = 53 g of potassium nitrate can dissolve. The saturated solution should have only 53 g of potassium nitrate in 50 g of water. NOT 106 g of potassium nitrate in 50 g of water. So answer is not D!

At 30 degree celcius, the solubility of the salt is 45 g for every 100 g of water. Thus, if we have 200 g of water, the maximum mass of potassium nitrate that can dissolve is 90 g. So 90 g of potassium nitrate in 200 g of water at 30 degree celcius will give a saturated solution.

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