Percent Composition of Hydrates
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Demonstrate proficiency in using the balance and Bunsen burner. Determine that all the water has been driven from a hydrate by heating your sample to a constant mass. Relate results to the law of conservation of mass and the law of multiple proportions. Perform calculations by using the molar mass.
Analyze the results and determine the empirical formula of the hydrate and its percentage by mass of water.
If the hydrated form of CuSO4 is heated then it will remove the water turning it to the anhydrous form then it will change from blue to white.
* Crucible and Cover
* Crucible and tongs
* Distilled water
* Stirring rod, glass
* Bunsen burner
* CuSO4, hydrate crystals
* Desiccator gauze
* Dropper or micropet
* Ring Stand
* Ring and pipe-stem triangle
* Weighing paper
1. Put on goggles and lab apron
2. Make sure equipment is clean, do not touch crucible after it has been heated, and let cool before weighing. 3. Place the crucible and cover on the triangle with the lid slightly tipped. Heat until the crucible is glowing slightly red. Record the mass 4. Using the spatula add 3g of copper sulfate hydrate crystals to the crucible and determine the mass. 5. Place the crucible on the triangle with the lid slightly tilted. Heat gently for 2-3 minutes and heat to crucible for minutes. You will observe a color change. Cool and record mass. 6. Repeat step 5 to verify the mass is correct.
7. Take part of your sample to rehydrate the crystals.
Observation: When the anhydrous form is reacted with water it returns back to its hydrated state. 8. Clean all lab materials and area
Mass of empty crucible and cover| 20.50g|
Initial mass of sample, crucible, and cover| 23.50g|
Mass of sample, crucible, and cover after first heating| 22.40g| Mass of sample, crucible, and cover after second heating| 22.39g| Constant mass of sample, crucible, and cover| 22.39g|
1. Why do you need to heat the crucible before using it in this lab? Why do the tongs used throughout this lab need to be especially clean? * To make sure the results are accurate because the crucible is being heated before all the other times that the mass is being recorded. * To make sure nothing gets in or on the crucible that could mess up the solution or change the mass.
2. Why do you need to use a cover for the crucible? Could you leave the cover off each time you measure the mass of the crucible and its contents and still get accurate results? Explain your answer. * You need to use a cover to make sure the contents does not splatter out when it is being heated * No, the crucible needs to be measured with the crucible and its contents each time you take the mass because some of the contents may be on lid and because it was used in the experiment and its data needs to be taken as well.
3. Calculate the mass of the anhydrous copper sulfate (the residue that remains after heating to constant mass) by subtracting the mass of the empty crucible and cover from the mass of the crucible, cover, and heated CuSO4, determine from the periodic table, to calculate the number of moles present. * 22.39g – 20.50g= 1.89g CuSO4
1.89g CuSO4/160g/mol = 0.01181 mol CuSO4
4. Calculate the mass and moles of water originally present in the hydrate by using the molar mass determined from the periodic table. * 23.50g – 22.39g= 1.1g H2O
1.1gH2O/ 18g/mol= 0.06167mol H2O
5. Explain why the mass of the sample decreased after it was heated, despite the law of conservation of mass. * Because when it was heated the water evaporated causing it to weigh less.
6. Using your answers from 3 and 4, determine the empirical formula of the copper sulfate hydrate. * 0.01181mol/0.01181mol=1
7. What is the percentage by mass of water in the original hydrate compound? * 23.50g-20.50g= 3gCuSO4•5H2O
1.11/3 x 100= 37% H2O
8. How much water could 25g of anhydrous CuSO4 absorb?
* x – .37x = 25g
x= 68g CuSO4•5H2O
9. When you rehydrate the small amount of anhydrous copper sulfate, what were your observations? Explain whether this substance would make a good indicator of moisture. * It went back to its blue color.
* The anhydrous form is a good indicator of moisture because it changes colors when wet.
10. Write equations for the reactions that occur when this mixture is heated. * CoCl2•2H2O+4H2O→CoCl2•6H2O
* CoCl2•6H2O→ CoCl2•2H2O+4H2O
11. Drawing Conclusions
A) Could the solid be a hydrate? Explain how you could find out? * Yes because the mass decreases after heating.
B) If the solid has a molar mass of 208g/mol after being heated how many formula units of water are there in one formula unit of the unheated compound? * 1.92g/208g/mol=0.00923mol of solid
12. Some electronic equipment is packaged for shipping with a small packet of drying material. You are interested in finding out whether the electronic equipment is exposed to moisture during shipping. How could you determine that? * You could ship it with the anhydrous form Copper Sulfate and if it has turned blue then the contents has been exposed to moisture and if it remains white then it has remained dry.