Exam 1
Suppose you start with 2.1000 g of cobalt(II) chloride hexahydrate. After you think that the water has been removed you record the mass of the solid remaining. It has a mass of 1.2587 g. Determine the mass percent water removed during the burning.
#2A how many waters of hydration prelab
Calculate the theoretical percent water in cobalt(II) chloride hexahydrate.
#2B. How many waters of hydration pre-lab
Compare the calculated percent water to the theoretical value.
#2C how many waters of hydration pre-lab
In another experiment you have 4.5000 g of a copper(II) sulfate hydrate with an unknown number of attached water molecules. After heating the hydrate, you have 3.3608 g of the anhydrous compound left. Determine the number of water molecules present in the formula of this hydrate. Write the formula.
#3 how many waters of hydration prelab
Calculate the limiting reagent, theoretical yield, and percent yield for the synthesis of alum
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Consider errors for the experiment and decide how the errors will affect the yield
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Given experimental data, similar to the data set you collected in lab, calculate the percent water in a hydrate and find the number of moles of water in the hydrate
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Outline a method for the synthesis of alum from aluminum.
1. Get and weigh aluminum (0.35 g) 2. measure out 2.0 g of KOH and 15 mL of H₂O 3. react in 250 mL beaker. 4. When done reacting filter to remove impurities 5. Put in ice bath to form crystals 6. Put through vacuum to dry out Alum powder.
Write an outline for measuring the volume of 1 drop of water.
1. Measure ~50 mL of water into an empty 100 mL beaker. 2. Weigh an empty 10 mL graduated cylinder. 3. Using a pipette put 1 drop of water into the graduated cylinder. 4. Weigh the graduated cylinder plus water. 5. Do calculations
5. Use the internet to find two uses of chromatography, describe each and include references.
1. Separating amino acids and anions 2.Fingerprinting
Outline a method for measuring the water content of CuSO4∙5H2O. List the equipment needed.
1. Set up bunsen burner 2. Measure empty crucible 3. Add ~2g of hydrate CuSO₄∙5H₂O and weigh 4. Heat on burner until the mass of the substance no longer changes 5. Weigh the anhydrous compound
How well a quantity agrees with the accepted value. Defined by percent error.
Accuracy
6. Describe how you will use chromatography to determine the identity of the unknown pen.
After chromatography is performed on the paper you compare the patterns. The mystery pen will be the same as one of the other pens, i.e. the colors, front, and direction will all be the same.
Based on the class data, which hydrate tested in this lab would you recommend to a company to be used as a desiccant? Explain your answer.
Based on the class data I would recommend that CoCl2 be used as a desiccant. I recommend this because for every 1 mole of CoCl2 there are 6 moles of H2O. This means that CoCl2 would absorb 2-3 times the amount of water as the other hydrates
Something used to keep moisture away from products that can be damaged by water
Dessicant
Errors that cannot be avoided or corrected.
Random
Errors that are inherent in the design of the experiment or in the instruments themselves.
Experimental Errors
Your lab mate determines that the volume delivered by a 5.00 mL pipet is actually 5.04 mL. Assuming that the pipet is correct, what is the percent error of your lab mate's measurement?
How Is Lab Equipment Used? #4
Errors due to carelessness of the experimentor
Human errors
Compounds that incorporate a specific number of water molecules into their structure
Hydrate
Chromatography
Method to separate a mixture of chemical compounds based on differing physical properties
The solvent/liquid where materials are dissolved and travel along until they are dropped
Mobile phase
State the results (as either the percent water or formula) of each hydrate your group tested. Compare these results to that of the other groups that measured the same unknown hydrate. Were there results comparable to yours? Why or why not? What can you conclude about the reproducibility of the experiment?
My group tested CoCl2 and MgCl2. I tested CoCl2. For CoCl2 our average % H₂O was 46.2%, which was the same as the other groups. For MgCl2 the average % H₂O ranged from 34.53%-59.10%. The results in MgCl2 were relatively comparable but I think that you would need to perform this experiment again to get a more accurate result. I can conclude that the reproducibility of the CoCl2 hydrate is very reproducible, since we both got the same average % H₂O, and the standard deviation was 3 however the reproducibility for MgCl2 could be improved since they had such a wide range of average % H₂O.
If the product you isolate is pure alum, do you think that obtaining a 100% yield is possible? Explain why or why not.
No, because Aluminum is water soluble, so some of your product is going to disassociate in water and will not be in your experimental yield.
Compares the experimental value and the accepted/exact/true value. Can be positive or negative.
Percent error
How well several measurements of the same quantity agree with each other
Precision
Indicate the one procedural step below that you SHOULD do in lab (the others are bad ideas). a. Stop the experiment after the solvent has only travelled 1.5 cm up the paper b. Spot your paper with the ink samples above the level of the solvent in the beaker c. Leave the top of the beaker uncovered during the experiment d. Use a pen to mark the starting and ending points of your different ink samples
Spot your paper with the ink samples above the level of solvent in the beaker.
Value calculated using a set of measurements or data and their average. Shows how much data points are scattered around the average
Standard Deviation
The surface where materials are deposited when they can't be supported by the liquid any longer
Stationary phase
What are the two types of experimental errors?
Systematic and random
Errors that are the same for each measurement with the instrument used.
Systematic errors
In this experiment, the stationary phase is what?
The chromatography paper
In this experiment, the mobile phase is what?
The ethanol/water solvent mixture
What was the mobile phase in who wrote the ransom note?
The solvent
Explain how chromatography can be used to identify an unknown compound
The solvent carries the specific ink up the chromatography paper based on the how polar the ink in the pen is.
3. What piece of glassware would you use to measure 25 mL of water? What would you use to measure 25.00 mL of water? Explain your choices.
To measure 25 mL of water you could use a beaker, or an erlenmeyer flask because these estimate to the 1's place digit. This would not be an accurate measurement. For 25.00 mL you want something that estimates to the hundredths place, so you are going to want to use something like a buret or a pipet.
Discuss which pieces of glassware are the most accurate and precise when given information about the %error and standard deviation of a set of volume measurements made with the various pieces of glassware
To measure 25 mL of water you have two options. You could use a large beaker, or an Erlenmeyer flask. You could use other glassware to measure the amount but 25 mL indicates that the 5 is the one estimated digit. You would use these pieces of glassware if you were not looking for an accurate measurement. On the other hand, if you wanted to measure 25.00 mL of water you would use a volumetric flask, a pipet, or a buret. I picked these three because they allow you to measure with certainty to the tenths place, and you can estimate at the hundredths place. With any other of the glassware options you would be estimating the tenths place, making your measurement less accurate.
How can you quantify your results of a chromatogram?
Using the Rf factor. (how far ink traveled/how far solvent traveled)
Part A: Describe how the best solvent was selected in your group. What were the criteria used for this selection? Why did you eliminate some of the solvent mixtures?
We chose the solvent that gave the most color variety as well as the one that all inks responded to.
After the reaction of aluminum and potassium hydroxide did you notice any impurities in your solution? What were these? Would these impurities affect your percent yield? If so, explain how, would your yield be lower or higher and why.
Yes, they were things like paint or coloring used on the aluminum foil. They would effect your percent yield because they would add to the total weight of your final product.
Explain why you had to cool your solution before isolating the alum. Does the temperature of the ice bath affect your percent yield? If so, explain how.
You must cool the solution so that crystals will form. If the temperature is not low enough then it will effect your yield because the crystals will not form and you will not be able to put them in the vacuum to dry.
Consider the following errors. How would each affect the calculated percent water in the hydrate? Indicate whether the percent would be higher, lower or unaffected. Explain your answers. a. the hydrate is heated too strongly and some splatters out of the container unnoticed b. after heating the hydrate several times, water still remains
a. Percent yield would be lower because you are losing mass when it leaves the crucible. b. Percent mass would be higher because the water would make the substance heavier. This would make it appear that you have more anhydrous solution than you really do.
