6 Exp. 6: Gravimetric Analysis of Limestone from Mackay, ID

Pre-Lab: Use BB to quiz on the following before the lab, but after reading the lab.

1. Using reactions 1 and 2, or 3 from your lab, if 0.250 g CO2(g) escape from your reaction flask after addition of HCl(aq), how many grams of CaCO3(s) are present in your limestone sample?

2. If your limestone sample from the previous problem weighs 0.80 g, what is the percent composition of CaCO3(s) in your sample?

3. Solid calcium sulfate is collected in a Büchner funnel and dried. The mass of the solid product is 1.03 g. How many grams of CaCO3(s) are present in your sample?

4. If your limestone sample from the previous problem weighs 0.80 g, what is the percent composition of CaCO3(s) in your sample?

5. What is the percent difference between the two methods of measuring a percent composition.

6. How will you avoid injury in this lab?

Introduction* This lab will be reported as a formal lab report. You should collect careful measurments and observations to help you complete the report. You may want to take pictures of your lab set up and observations to include in your report (optional).

Gravimetric analysis, a quantitative analysis technique, can be applied to various processes including determination of the composition of minerals or rock to quality control measures quantifying finished products and raw materials in industry. Used to measure the unknown mass of an analyte, it consists of precipitating the analyte as a known compound. The compound can then be isolated and weighed, yielding the percent composition of that analyte, or ratio of mass of the analyte to the entire sample. In a related process, removing an analyte can yield the percent composition of the compound by subtraction. With either method, a balanced equation relates either the precipitate or volatile compound that is removed to the entire sample. In this experiment, you will perform both a gravimetric analysis of a precipitate and subtraction of a volatile compound to find the percent composition of calcium carbonate, CaCO3, in a sample of limestone from Mackay, Idaho, a town sometimes called the ‘top’ of Idaho, due to its proximity to Mt. Borah and the Lost River Range.

What makes the geology of this region more interesting is that the limestone that you will analyze formed in what was anciently a warm, shallow sea, allowing invertebrates to absorb the minerals from the ocean water in the form of shells, corals, algae, and other creatures. Limestone represents an important sedimentary rock with a biological origin. CaCO3 is of particular interest, as solubility decreases with increasing temperature. Its solubility properties allow it to dissolve to create caves and sink holes as well as precipitate to form stalactites and stalagmites.

In this laboratory, you will analyze two samples, first via subtraction and then via gravimetric analysis for a total of four data points. You will then compare the results from two different methods in order to determine the percent composition of your analyte.

% composition =             Equation 1

The following reactions will be useful as you perform your analysis. Your analyte of interest, CaCO3(s) reacts readily with HCl, producing carbonic acid (Reaction 1) which then decomposes to CO2(g), (Reaction 2). The reaction of CaCO3(s) and a strong acid such as HCl is often used by field geologists as a test for limestone or other carbonates.

CaCO3(s) + 2HCl(aq) à CaCl2(aq) + H2CO3(aq)                                      Reaction 1

                                    H2CO3(aq) à  CO2(g) + H2O(l)                                                 Reaction 2

                        ——————————————————-

net reaction:   CaCO3(s) + 2HCl(aq) à CaCl2(aq) + CO2(g) + H2O(l)                              Reaction 3

By carefully measuring the mass of your sample and acid before and after the effervescence of CO2(g) occurring with the dissolution with HCl, you will be able to determine the amount of CaCO3(s) in your original sample. This determination is possible based on the reaction stoichiometry from reactions one and two. This measurement depends on an assumption. What is that assumption? Be sure to include it in your formal report.

In addition, you will quantify the amount of CaCO3(s) through precipitation of the Ca2+ ion (Reaction 1) as CaSO4(s).

CaCl2(aq) + (NH4)2SO4(aq) à CaSO4(s) + 2NH4Cl(aq)                           Reaction 4

By isolating the Ca as CaSO4(s), you will be able to quantify the Calcium present in the original sample as CaCO3(s). This quantification also depends on an assumption. What is this assumption? Be sure to include it in your formal report.

By finding the percent composition through two different methods, the evolution of CO2(g) and the precipitation of CaSO4(s), a comparison is possible. Would a percent error or percent difference be more appropriate? See lab one for examples and formulae.

Expt.

Materials and Procedures

Limestone

Vacuum Filtration with Trap

Buchner Funnels with removable filtering surface

Quantitative Filter Paper Q5

3.00 M Hydrochloric Acid (HCl)

2—125 mL Erlenmeyer flask

150 mL Erlenmeyer flask – 2

Methyl red indicator

Ammonium sulfate (NH4)2SO4

2.00 M aqueous ammonia (NH3) also labeled Ammonium Hydroxide (NH4OH)

Transfer Pipette

Glass stir rods

Watch Glass – in your Drawer-2

Drying Oven Set at 110 °C

Procedure Carbonate in Limestone

*Some notes on careful weighing and measuring.

The validity of your data in this lab depends on careful measurement and recording technique. The changes in mass are minute; even small errors will affect your calculated results. Writing your formal report without reliable results will be difficult.

• Plan your data collection before you start. Understand why you are recording a measurement.
• Use the same balance for a sequence of measurements. Be sure to tare the balance after another group uses it.
• Choose whether to record and subtract the mass of glassware used or to tare the balance with the glassware and record the mass of reagents.
• Plan ahead with the final drying step. You must choose to attempt to scrape your dried product from your Büchner funnel, or you must record the mass of your dry Büchner funnel and filter paper before you filter. Each funnel and paper have a different mass.

Part 1

1. Obtain enough limestone sample so that you have two separate rock samples weighing ~0.70 grams to 0.80 grams and record the exact mass in your lab book. You will want to choose small pieces. They are already broken up for you.  The bigger your pieces the longer it will take to dissolve your limestone.

2. Add approximately 15 mL of 3.00 M HCl to each of a clean and dry (important!) 125 mL Erlenmeyer flask(s). Label your flasks with a Sharpie or label tape before weighing them. Record the total mass of the container with the HCl in your lab book.

3. Add the limestone to the flask with the HCl. Swirl gently to mix the solution and allow the reaction to proceed until bubbling has stopped. If you are unsure if the reaction is complete, wait a bit and re-weigh to see if the mass is still changing. You may still see some solids left in the flask and it may be slightly dark. This is the portion of the rock that is not calcium carbonate and is not involved in the reaction.

Figure 3. Reacted Limestone and HCl

4. Record the mass of the reacted limestone and HCl.

5. Keep this solution as you will use it in Part 2.

6. If your sample has any undissolved solids, carefully filter the solution using gravity filtration as depicted.

Diagram 1: Gravity Filtration

The following sample data table may be helpful as you record your data and perform your stoichiometric calculation: Be sure to include sample calculations in your formal report. Analyze reactions 1-3 in order to discern your reaction stoichiometry.

Figure 4. Gravity Filtration set up

Part 2

*Helpful notes about precipitation:

Although your precipitation occurs based on the mole ratio of Ca2+ and SO42-, adding excess water will inhibit precipitation. Use small volumes of water for any rinse steps that occur before precipitation.

1. Add 10 drops of methyl red indicator to the filtrate from part 1. The sample, stored in a 125 or 150 Erlenmeyer flask, should turn a pink color.

2. While stirring with a stir rod or swirl flask (you can also use a stirring plate), add at least 1.5 grams of Ammonium Sulfate (NH4)2SO4 to the flask. Continue stirring until the ammonium sulfate completely dissolves.

3. Using a 3 ml transfer pipette, slowly add 20 ml of 2.00 M Ammonium Hydroxide NH3OH, Continuing adding the Ammonium Hydroxide drop wise using a 3 ml transfer pipette until the color just shifts from pink to pale yellow. It is important not to overshoot this color change the change will occur with one additional drop of Ammonium Hydroxide. The color change should be maintained with stirring or shaking. You can check the pH with pH paper.  With the addition of the base the pH should change from 4.4 to 6.4, with the pale-yellow color.

See the sequence below. 

            Figure 5: Precipitation of CaSO4(s)

1. Dissolved (NH4)2SO4 B.  Addition of NH4OH precipitate forming  C.  Color change with one drop of NH4 OH
2. Obtain a clean and dry Büchner funnel and piece of #5 filter paper.

5. Consider the goal of filtering and drying Calcium Sulfate precipitate. What is the final measurement you will use in your calculations? This will inform your next few steps.

6. Record the mass of the dry filter paper to be used in your Büchner funnel. You won’t be able to dry your sample in your funnel, so you may need to weigh an evaporating dish or watch glass. Plan a few steps ahead. Would you get the most accurate result by weighing your dry precipitate and filter paper in a tared weigh boat? If so, you do not need to weigh an evaporating dish or watch glass. Will you get the best results by scraping your dry precipitate from your filter paper? If so, you don’t need to weigh your filter paper. Adjust your data table to reflect your process. *If you fail to weigh your filter paper in this step, weighing a second paper and assuming the masses are the same will introduce substantial error.

A Büchner filtration system has been set up for you. Your analyte has now precipitated as a solid, Be sure to empty the filtrate from your Buchner set up into the waste container when finished.

7. Place the weighed filter paper in the Büchner funnel. Wet the paper thoroughly with distilled water. Turn on the vacuum to allow the paper to be sucked onto the funnel to ensure a good seal.

8. Collect the crystals by pouring the solution onto the filter paper in the funnel. Do not disturb the crystals on the filter paper or you will break the filtration vacuum. Wash any product left in the flask into the funnel with cold distilled water; rinse your stirring rod as well. Remember, any product you lose here will affect your mass percent.

9. Wash the precipitate with distilled water and allow the vacuum to pull air through to dry your sample for a few minutes.

10. *(Optional: If your sample will dry in the oven between lab periods, you may skip this step) Allow the filtration to continue until you see no more water droplets fall from the funnel. Turn the vacuum off water off. Rinse the product with a little acetone from an acetone rinse bottle to drive off excess water. Wait a minute and turn the vacuum on again to pull the acetone through the filter.

11. Place your filter papers and precipitates in the oven using a watch glass. Be sure to label your sample (prior to weighing).

12. To ensure that your sample is dry, record the mass and return to the oven. After another 10 minutes in the oven, re-weigh. If your sample mass is still changing, it is not yet dry. Once you are satisfied that your sample has dried, let it cool and record the mass in your lab book. During successive weighing, the mass of precipitate should differ by no more than 5%.

Check that you have your own sample and not some else’s sample.

The following sample data table may be helpful as you record your results. Be sure to include sample calculation in your final report. Be sure to refer to reactions 1-4 for reaction stoichiometry. You may use two Buchner tops available or may have to filter and dry during successive lab periods.

Your Report

You will use the data and observations from this lab to complete your first formal lab report. Please see the Formal Lab Documents tab included in your BB shell, including the requirements found document, How to Write a Formal Lab Report found in your Formal Lab Documents file. To help you reference the document, use this incomplete list of requirements.

• A journal sourced application of the lab technique to be included in your Introduction
• Any assumptions made in your Introduction
• Any formulas or reactions used in your Introduction
• Any deviation from procedure as well as actual masses and concentrations in your Experimental section
• Tabulated raw data in your Results and Discussion
• Sample calculations in your Results and Discussion
• Discussion of any differences in % composition between samples first with gas evolution then with precipitation in the Results and Discussion section.
• Discussion of any differences in % composition within technique 1 and technique 2 in the Results and Discussion section
• Statistical treatment and discussion of any differences in average % composition between technique 1 and technique 2 in the Results and Discussion section including % error or % difference as appropriate (see your student appendix).
• Statistical treatment and discussion of the deviation between the four data points you obtained in the lab. Use a 95% confidence interval as described in your lab appendix. Discuss the reported error in your Results and Discussion section. (see Appendix IV)
• Findings should be listed in the Abstract, Results and Discussion, and Conclusion and discussed in the Results and Discussion section.
• Address whether or not the assumptions listed in the introduction were validated or were not validated in the Conclusion section.
• Reference citations (not URL) after the Conclusion
• An originality report will be automatically generated upon your submission of the exam and will be visible to you. Please note that in science writing, direct quotations aren’t used. You must, therefore, convey the ideas and theories as well as past work cited by literature without plagiarizing another’s work. The originality report will alert me to similar reports across sections and semesters. It is reasonable to report the same measurements and numerical data as your lab partner. Duplicated work of less than 15% is generally accepted in journal writing. Unoriginal content of greater than 25% is generally considered plagiarized in journal writing. Reproduced writing, except as described previously, whether from a literature source or another student represents plagiarism, and the first instance will result in a zero grade for the assignment without the opportunity to re-write the assignment.

Your formal report (6) and informal report for lab seven (7) Testing the Solubility Rules will be turned in via your lab Blackboard shell during the week prior to final exams. Please plan accordingly.

References

Adapted from Gregg Dieckmann and John Sibert’s “Percent Composition from Gravimetric Analysis: Calcium Carbonate in Texas Limestone.” An Atoms First Approach to the General Chemistry Laboratory. New York: McGraw-Hill. 201

British Geological Survey 1998 – 2017 (c)NERC mailto:www-bgs@bgs.ac.uk. How is limestone formed? https://www.bgs.ac.uk/discoveringGeology/geologyOfBritain/limestoneLandscapes/whatIsLimestone/howFormed.html (accessed Jun 30, 2020).

Personal Communication. 2015.  Dr, Gregg Dikeman University of Texas at Dallas dieckgr@utdallas.edu

Quantifying the Composition of Limestone – Chem21Labsm University of Kentucky, Lexington

www.chem21labs.com/labfiles/UKY_111_Exp4.pdf