Chemistry 140

Please have the following pages ready before class on Wednesday, October 30. Note that the different parts will be standard divisions in all lab writeups. For this particular writeup, please write an abstract and paper-clip it to the front of your individual writeup. The abstract and the carbon-copy pages of the write-up is due in class on Wednesday, November 6.

Graphs are a critical part of describing the results of most experiments. In this lab, you will use a graphing program, such as Excel, to plot the results of this experiment in order to determine the stoichiometry of a reaction with an unknown precipitate. However, this is not the main purpose of this lab.

In this experiment, you will be given two solutions, one of calcium acetate and one of sodium carbonate. You will determine the identity of the precipitate in this reaction by reacting various amounts of the solutions and weighing the precipitate, then comparing your precipitate mass versus various theoretical yield masses.

For background material, the text sections 3.8, 4.3 and 4.7 may be helpful.

Your name, your partner’s name, date of experiment

Lab 3: Determination of an unknown precipitation through limiting reagents

Part 1. Purpose

Please write a sentence or two stating the goal of this experiment, based on the information on above. Write the full chemical equation that describes what is going on in this experiment. Also write the net ionic chemical equation for what is going on.

Part 2. Materials and methods

Chemical list: ____ M sodium carbonate; _____ M calcium acetate

Sketch the setup as you are using it and label the various pieces of equipment. The picture below may help.

Part 3. Procedure

1. Obtain two clean 250 mL beakers and half-fill one with the sodium carbonate solution and half-fill the other with calcium acetate solution. Label each beaker with lab tape so you know which is which.

2. Obtain two clean 10 mL pipets. Label one for use with sodium carbonate and the other for use with calcium acetate. Do not cross-contaminate your solutions.

3. Obtain seven clean 50 mL beakers and label them “1” to “7”.

4. Obtain seven pieces of filter paper circles and label them “1” to “7” along the edge with a pencil, together with your initials. Record the dry mass of each piece (just because they look the same don’t mean they weigh the same). Handle the filter paper with tweezers from this point on.

5. Obtain a clean 250 mL Erlenmeyer filter flask and a clean Büchner funnel threaded through a rubber stopper. Also obtain a length of rubber tubing, a ringstand and a clamp. Clamp the flask and funnel to the ringstand as shown in the materials and methods section; make sure the rubber stopper is snugly seated on the flask. Connect the hose between the aspirator at the sink and the flask nozzle. Connect an aerator on the end of the aspirator.

6. Use the pipet and bulb to dispense the following amounts of sodium carbonate solution into beakers “1” through “4”, respectively.

Beaker #	1	2	3	4
Na2CO3 volume (mL)	2.0	4.0	6.0	8.0
Ca(C2H3O2)2 volume (mL)
Total volume (mL)	10.0	10.0	10.0	10.0

To use the bulb for suction, first make sure the bulb is compressed by squeezing the top of the bulb area.

Then, squeeze the valve on the main stem to draw in fluid.

Finally, squeeze the valve on the side stem to expel the fluid. Use this valve for getting rid of excess fluid as well.

7. Add the appropriate amount of calcium acetate solution to each beaker, so that the total volume in each beaker is 10 mL.

8. Note appearance changes in the solutions in your data table; use relative terms like “lightly cloudy” or “dark red” and note the relative amount of precipitate (e.g., “most”, “least”, etc.) in each.

9. Place filter paper circle “1” in the Büchner funnel pencil-side down and vacuum-filter the contents of beaker 1. Make sure that there is an aerator fitting on the aspirator before turning on the water. Remember to wet the filter paper with a squirt (wash) bottle of distilled water.

10. Use distilled water to remove any solid residue from the beaker; be sure to wash it over the filter funnel. With a spatula, and without tearing the filter paper, make sure that the solid material is on the filter paper (scrape any solid residue off the sides of the funnel). If the filtrate looks very murky, break the vacuum and refilter the filtrate. Repeat until the filtrate is clear.

11. Allow the solid material to dry for a few minutes while the vacuum is still on.

12. Remember to break the vacuum at the flask nozzle! Empty the (basically clear) filtrate in the flask into the sink.

13. Carefully (so it doesn’t tear) remove the filter paper from the funnel and place it on a paper towel. Allow the filter paper to dry.

14. Repeat for the other three beakers; use the appropriately numbered filter paper for each beaker. Place the whole piece of towel paper (labeled with your name) on the side table; be careful not to upset the towel! Skip to step 16.

15. After the weekend, weigh the dry filter paper containing the precipitate; record the mass. Determine the mass of the precipitate for each beaker.

16. Based on your observations of the “most” and “2^nd most” precipitate-containing beakers, determine between what two volumes of the sodium carbonate solution the maximum amount of precipitate will fall. You will make three more mixtures (beakers 5, 6 and 7) of the two solutions (same total volume) at half-milliliter increments of the sodium carbonate solution within that interval. For instance, if you determine that the beaker 3 and beaker 4 mixture had the highest masses of precipitate, then you would put 6.5, 7.0 and 7.5 mL of sodium carbonate solution into beakers 5, 6 and 7, respectively, then fill each to 10 mL with the appropriate volume of calcium acetate solution.

17. Repeat the vacuum-filtration for beakers 5, 6 and 7; once dry (after the weekend), weigh the filter paper. You may place these filter paper circles on the paper towel, or you may place them on a different paper towel. In any case, label the paper towel(s) with your name!

Waste disposal — All solutions are safe to pour down the drain; use sufficient tap water to make sure the salt solutions are diluted.

Part 4. Original data

Sample table:

Beaker #	1	2	3	4
Na2CO3 volume (mL)
Ca(C2H3O2)2 volume (mL)
Appearance/amount of precipitate
mass of dry filter paper (g)
mass of dry filter paper and precipitate (g)
mass of precipitate (g)
millimoles of Na2CO3 reacted
millimoles of Ca(C2H3O2)2 reacted
millimoles of NaC2H3O2 expected
millimoles of CaCO3 expected
grams of NaC2H3O2 expected
grams of CaCO3 expected
mass of precipitate indicates the identity of the precipitate is

Note that the double underline separates part 4 data collection from part 5 calculations.

You will need a similar table for beakers 5 through 7.

Part 5. Calculated results

Show a sample calculation for one of the beakers, showing how you derived the expected values, and how you decided on the identity of your precipitate.

Using any spreadsheet/graphing program, make a plot of sodium carbonate volume (x-axis) versus mass of precipitate (y-axis). Make sure the axes are labeled (along with units) and that the graph has a title, such as “Precipitate mass as a function of sodium carbonate volume for lab 3, Chemistry 140”.

The points on your graph should slope up towards the right, then slope down after reaching a maximum. The “ascending series” will be the points that fall in the part of the graph that slopes upwards. The “descending series” will be the points that fall in the sloping downward portion.

Use the functions available on the program to draw the best-fit line for the ascending series and another for the descending series. If it is possible to display the equation for both lines, do so; in any case, determine the volume of the sodium carbonate solution that generates the maximum amount of precipitate.

Part 6. Group results

Once your group has obtained the masses of the dry precipitate, write the maximum value on the board. Eliminate outliers, if necessary, and calculate a mean and standard deviation of the maximum precipitate mass for the class.

Part 7. Questions

1. How do you know that the precipitate cannot be either reactant molecule?

2. In the ascending portion of the graph, which reactant was limiting? In the descending portion of the graph, which reactant was limiting? What is the significance of the intersection point of the best-fit lines on the graph?

3. Given the concentrations of the original solutions, what is the expected value of the volume of sodium carbonate solution that would lead to the maximum amount of precipitate? Did your value for the volume at the intersection of the best-fit lines agree with the expected value? Calculate a percent error for your value, based on the expected value. Is this error related to accuracy or precision?

4. Explain if the class mean for the maximum mass of the precipitate was closer to the expected value for the maximum mass or not. Did the range described by the standard deviation include the expected maximum mass?

5. The solubility of the precipitate is 0.014 g/100 mL of water at room temperature. How would this affect your results (i.e., would this lower or raise the precipitate mass and how)? Does this therefore qualify as a systematic error?

Part 8. Conclusion

First sentence: “We determined that the identity of the precipitate in this reaction was _________ . “

Describe how you could tell it was this chemical rather than the other possible product.

Mention any significant random and/or systematic errors and how they could be fixed or not fixed next time. Question 5 might be of some help here.

Mention: “The class mean for the maximum amount of precipitate was _________ ± _________ g, which is (consistent/inconsistent) with our result.”

Last sentences: Did you accomplish the goal of the experiment? How confident are you of your results? For the confidence part, you may wish to refer to question 3.

Abstract

As usual, a short (less than 100 word) summary of the major result of your experiment, and the method by which you achieved this result. Report both your own result (the identity of the precipitate and your maximum yield of precipitate) and the class mean and standard deviation for the maximum precipitate. Report also the percent error between the class mean and the maximum expected yield. Report the source of any significant errors in your own result.