A mixture of 100 g of K2Cr2O7and 200 g of water is stirred at 60°C until no more

Question

A mixture of 100 g of K2Cr2O7and 200 g of water is stirred at 60°C until no more of the saltdissolves.The resulting solution is poured off, leaving theundissolved solid behind. The solution is now cooled to 20°C.What mass of K2Cr2O7 crystallizes from the solution during thecooling?

Please EXPLAIN youranswer. A mixture of 100 g of K2Cr2O7and 200 g of water is stirred at 60°C until no more of the saltdissolves.The resulting solution is poured off, leaving theundissolved solid behind. The solution is now cooled to 20°C.What mass of K2Cr2O7 crystallizes from the solution during thecooling?

Please EXPLAIN youranswer.

Explanation / Answer

The amount of solute that will dissolve in a given amount ofsolvent depends on the temperature. A solution is said to be"saturated" when it contains the maximum amount of solute that canbe dissolved at that temperature. By changing the temperatureof the solution from 60 degrees to 20 degrees, you are decreasingthe amount of solute, K2Cr2O7,that can be dissolved. The difference between what dissolvesat 60 degrees and what dissolves at 20 degrees is the amount thatcrystallizes out of the solution. To solve the problem, you need to know, first of all, how muchpotassium dichromate will dissolve in the 200 g of water at 60degrees C. According to my reference book, Lange's Handbookof Chemistry, 14th edition, 100 g of water at 60 degrees C willdissolve a maximum of 45.6 grams ofK2Cr2O7. Since you havetwice as much water, you can dissolve twice as muchK2Cr2O7. You can easilyfigure out how much that is. The solution is poured into a new container and allowed to cool to20 degrees C. At this cooler temperature, a saturatedsolution of K2Cr2O7 contains only12.3 grams of the solute per 100 grams of water. Since youhave 200 g of water, you'll have twice as muchK2Cr2O7, or 24.6 grams remainingdissolved at the lower temperature. No more than that willremain dissolved. The rest of it will crystallize out. The previous responder gave a different number for the solubilityat 60 degrees. That's because he or she used the data for aslighly different substance, K2CrO4. (potassium chromate--your problem was for potassiumdichromate,K2Cr2O7). I hope that this helps. --Sara

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