Water in a Solid Lab Fall 2014 PDF

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Lab assignment...

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Student Name____________________________ Partner’s Name___________________________ Date___________________________________

College Chemistry I 2045C Determination of W Water ater in a Solid Lab-SL Objective: In this lab, you will be using quantitative analysis to determine the percent by mass of water in a solid sample. Materials: Bunsen Burner Ceramic Tile Ring Stand

Crucible and Cover Heating Triangle Sample of Hydrate

Crucible Tongs Iron Ring

Background: The accurate determination of the composition of substances is essential in modern technology and important in our everyday life. Typical problems in which analytical chemistry plays a role are determining the compositions of lunar and other extraterrestrial samples, monitoring harmful levels of pollutants in the atmosphere above our cities and water supplies, maintaining quality control over drug and food products, detecting trace impurities in ultra pure semi-conductor materials (transistors, diodes, etc.) and making clinical investigations which determine the nature and concentration of materials in biological fluids. As can be seen from the examples given, analytical determinations may be either qualitative or quantitative in nature. Qualitative analysis is aimed at determining whether or not a substance is present; quantitative analysis determines the amounts of known substances present. This experiment will introduce you to precision weighing and quantitative analysis techniques. You will determine the percent by mass of water in a sample of solid. The solid contains one or more hydrates – salts that crystallize with a definite number of water molecules in each formula unit. For example, barium chloride dehydrate, BaCl2 ∙2 H2O, is a hydrate containing two molecules of water (water of crystallization) for each BaCl2 unit. You will heat the solid to drive off the water of crystallization, and from the loss in mass, calculate the percent of water that was present. Procedure: 1. The first thing you need to do is check your crucible and cover for any breaks or hairline cracks. Look at them carefully. To check for hairline cracks, tap the crucible with your finger. You should hear a ping, not a thud. If you are unsure of any possible cracks, check them with your instructor. 2. Set up your ring stand with the iron ring and heating triangle. Practice using your tongs to take the crucible on and off the triangle. Your INSRUCTOR will demonstrate the use of tongs. Practice placing the lid on the crucible and taking it off again. It is best to become adept at these procedures before you have a flame and sample to worry about. (Please refer to photo on the following page. page.)) 3. Clean and dry the crucible and cover thoroughly. Place the empty crucible with the cover ajar on the triangle on the ring. Heat gently for a few minutes. Adjust the Bunsen burner for a hot, blue flame. The inner blue cone should come about 2-3 cm above the burner. Heat the crucible strongly for 5 more minutes, with the crucible about 2 cm above the inner blue cone of flame. Note: The bottom of the crucible may glow a dull red during this time. 4. Turn off the burner. Note: From this point on, your fingers should NOT touch the crucible! With the tongs, move the crucible and cover to the ceramic tile to cool. INSTRUCTOR will demonstrate the proper way to determine when the crucible and cover have cooled sufficiently. 1

5. Once the crucible and cover have cooled, take them to be weighed. You will need to use your tongs and carry them over your ceramic tile as an added security precaution. 6. Make sure the balance is zeroed. Weigh the crucible and cover to the nearest milligram (0.001 g). 7. Obtain a known solid hydrate from your instructor. Weigh out between 2.2 and 2.5 grams of your sample on weighing paper to the nearest milligram. Place the sample in the crucible and reweigh the crucible with the sample to the nearest milligram. 8. Place the crucible on the triangle on the ring with the cover ajar as you did before. Heat the sample gently for a few minutes, and then again with the hot blue flame for 15 minutes. Turn off the burner, and use your tongs to move the crucible and sample and cover to the tile. Allow to cool to room temperature. 9. Measure and record the mass of the crucible, cover, and sample residue to the nearest milligram. 10. Repeat steps 8 and 9 above. Compare the mass in step 10 with the mass in step 11. Show these masses to your instructor at this point. Your instructor will determine if you need to repeat steps 8 and 9 one more time or not. Once your instructor has signed off on measurements, you are to consider your last measurement of mass to be your final weight.

12. Soak the crucible and cover in tap water in a beaker to remove the residue. If the residue remains, see your instructor for further clean-up instructions. When the crucible is clean, rinse it with distilled water, and dry it before placing it back in your drawer. Clean up your entire lab area. Make sure your balance is set back to zero. You may not leave lab until your instructor has signed you out. Analysis: All Calculations Must Be on Separ Separate ate Paper and All W Work ork Must Be Shown. DATA Sample Hydr Hydrate ate Experimental Data: Mass of Empty Crucible and Cover:

________________________ g

Mass of Crucible and Cover and Sample:

________________________ g

Mass of Crucible and Cover and Residue After the First Heating:

________________________ g 2

Mass of Crucible and Cover and Residue After the Second Heating:

________________________ g

Mass of Crucible and Cover and Residue After the Third Heating if needed:

_______________________ g

RESULTS Mass of Sample Hydrate:

_______________________ g

Mass of Water in Sample:

_______________________ g

% Water by Mass:

_______________________ %

Ask your instructor what your sample hydrate was so that you can compare your experimental data to the theoretical data. Sample Hydr Hydrate ate Theoretical Data: Sample Hydrate Formula:

__________________________

Mass of Sample Hydrate Based on Formula:

__________________________ g

Mass of Water Based on Formula:

__________________________ g

% Water by Mass According to Formula:

__________________________ g

Calculation of % Error: % Error = (% Water in Your Sample) – (% Water According to Formula) x 100 (% Water According to Formula)

Pos Postt Lab: Show all W Work ork on Separate P Paper aper for the ffollowing ollowing three problems. A student placed 2.306 g of CoSO4 ∙ xH2O in a crucible and heated it to a constant mass of 1.271 g. 1. What is the mass of water lost? 2. What is the percent of mass of water in this hydrate? 3. Calculate the volume of “x” in the hydrate. This is a tougher problem. Be sure to show your work and reasoning.

3

You may answer the following two pro problems blems below or on separ separate ate paper paper.. W Write rite in complete sentences and pa pay y attention to gr grammar ammar and spelling. 1.

While carrying the crucible and dried sample to the balance for final weighing in the procedure, a student unknowingly spilled some of the sample out of the crucible. What effect will this have on the student’s calculated value for the percent water? Will it make the value too high or too low? Explain below.

2.

In the experiment, you were instructed to allow your sample and crucible to cool to room temperature before you complete your final weighing. A conscientious student allowed the crucible to cool for a full week with the cover ajar so that the sample inside was exposed to the air. Can you identify any problems with might be cause by this change in procedure? Would the apparent percent water be too high or too low? Explain below.

Instructor’s Initials: _________________________ 4...