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E^ x^ p^ e^ r^ i^ m^ e^ n^ t
Quantifying the
Composition of
Limestone
Lecture and Lab Skills Emphasized
- Determining the percent composition of a compound in a substance.
- Applying gravimetric analysis.
In the Lab
- Students will work in pairs.
- Parts must be completed in order.
- Record your procedure and original data in your lab notebook along with your calculations.
- Report data collected and subsequent calculations to www.chem21labs.com.
- All equipment should be returned to the correct location after use.
Waste
- Pour the solutions in the waste container.
- Solids can go into the trashcan.
Safety
- Gloves and safety goggles are mandatory when anyone is performing an experi- ment in the lab.
- Hydrochloric acid (HCl) is toxic by ingestion and inhalation and corrosive to skin and eyes; avoid contact with body tissues. You are using very concentrated HCl.
- Aqueous ammonia (NH 3 ) solution is corrosive and contact with skin or eyes should be avoided. Work in a well-ventilated area.
- Take care in handling solid reagents to avoid inhalation and eye exposure. Always use a brush to clean the balance.
E^ x^ p^ e^ r^ i^ m^ e^ n^ t^4 •^ Quantifying the Composition of Limestone
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- Long pants, closed-toed shoes, and shirts with sleeves must be worn at all times. Clothing is expected to reduce the exposure of bare skin to potential chemical splashes.
- Always wash your hands before leaving the laboratory.
Additional information can be found at http://genchemlab.wordpress.com/4-empirical-formula.
Your supervisor has been asked to have multiple samples of limestone studied to determine how much calcium carbonate is present in some samples collected by a local geologist. The geologist wants to use this data to see if she can determine where unknown samples came from.
Geochemistry of Limestone Limestone comprises approximately 10% of all sedimen- tary rocks exposed on the earth’s surface and is primarily biological in origin. Typically, limestone forms from the lithification (the process of turning loose sediments into rock) of marine organisms like shells, corals, and algae on the ocean floor. Limestone formation can also come from precipitation of dissolved calcium carbonate. This commonly occurs in clear, shallow, warm water. Limestone is common in many parts of the U.S. and makes up more than 50% of surface rocks in Kentucky. 1 What does that tell us about the state of Kentucky mil- lions of years ago?
Limestone is composed of the mineral calcite, better known to chemists as calcium carbonate, small amounts of clay, silt, chert (silica, SiO 2 ), and dolomite (calcium- magnesium carbonate, CaMg[CO 3 ] 2 ). 2 To be called limestone, the rock has to have a composition of over 50% dolomite and calcite, though the exact percentages vary widely. The limestone available commercially is over 80% calcite and dolomite. Traditionally, the chalk in chalkboard was produced from limestone. However, the chalk used on chalkboards today is being replaced by gypsum (calcium sulfate, CaSO 4 ). Limestone is still used in AgLime and Lime, Portland Cement, and as stone used in construction projects. Quarries across the state and as nearby as the Lexington Quarry Company in Nicholasville and the Caldwell Stone Company in Danville have discovered calcite crystals along veins they were mining.^3
Figure 4.1. Limestone.
When examining your rock samples, you may notice variation in color. The color of limestone can range from bright white to light gray, frequently an indication on the purity of calcium carbonate. Impurities found in limestone include iron oxides, the yellow and brown shades, and carbon impurities from plant material, dark gray to black colors. Magnesium, silicates (SiO 4 –4), manganese, iron, titanium, aluminum, sodium, potas- sium, and sulfur found within limestone generally came from the seawater present when the limestone was originally created.
Limestone fizzes (or effervesces) with the addition of acid due to the release of carbon dioxide gas. This occurs in a two-step process. Acids will react with the calcium carbonate producing carbonic acid (H 2 CO 3 ). Then in the second step, carbonic acid rapidly decomposes to form carbon dioxide.
Step 1: CaCO 3 (s) 2HCl(aq) → CaCl 2 (aq) H 2 CO 3 (aq)
Step 2: H 2 CO 3 (aq) → CO 2 (g) H 2 O(l)
Net reaction : CaCO 3 (s) 2HCl(aq) → CaCl 2 (aq) CO 2 (g) H 2 O(l)
Natural acids within ground water can dissolve lime- stone in the ground, resulting in caves commonly seen in Kentucky and around the U.S.
E^ x^ p^ e^ r^ i^ m^ e^ n^ t^4 •^ Quantifying the Composition of Limestone
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Gravimetric Analysis In order to determine what elements are present, chemists often combine the sample with a compound known as a precipitating agent. Precipitating agent combines selectively with the element of interest in the dissolved sample. Can you identify the precipitating agent used in this experiment? The precipitate is then isolated and weighed to quantify the substance of inter- est. This method is known as gravimetric analysis. Two different gravimetric analysis methods are used in this experiment; can you identify them?
Ideally when choosing a precipitating agent, the prod- uct would need to be insoluble, easily filterable, very pure, and with a known composition. While you may not be able to meet all of these criteria, it is important that you use good experimental technique to ensure solid results. Good technique in chemistry lab refers to your ability to perform basic laboratory skills, including cleaning and drying glassware, following safety proto- col, accurately using glassware and other equipment, and completely transferring materials from one con- tainer to another. These are just a few of the techniques that you will use in the lab, all of which will affect the quality and quantity of the product you produce.
Filtration To perform a gravimetric analysis, you will need to be able to filter the solids formed in your reaction out of solution. Filtration is used to separate components which are soluble (dissolve in a solvent) and insoluble (do not dissolve in a solvent). A solvent can be just about anything, regardless of the phase. We tend to think of solvents as liquids, such as water, but solvents and solutes (the species which dissolve in the solvent) can be in any phase and it is not necessary for them to be in the same phase. Filters in chemistry lab work like filters in cars or your house, or the colanders in your kitchen. When cooking pasta, we pour the pasta and water through a colander, which is our filter. The holes are large enough for the water to go through but too small for the pasta. In chemistry, we are usually talk- ing about much smaller things so we use filter paper which has holes like a colander, only they are much, much smaller.
To filter out the solids of the solution you produce in this lab, you will use two different techniques. The
simplest technique uses gravity to filter the solution through filter paper (Figure 4.2). This system works well when the solids are large and you are interested in the solution. When you have small crystals that you want to keep, vacuum filtration would be more appro- priate, as it uses a vacuum to pull suction on a special piece of glassware known as a filter flask with a Büchner (pronounced Byook-ner) funnel to hold the filter paper (Figure 4.3). The difference between the two methods is like the difference between picking up a large bag of peas spilt on the floor by hand or by using a vacuum cleaner. We are using the power of suction to pick up the peas, just like we’ll be using the suction to pull the liquid through the filter paper. In the lab, suction can be generated by a vacuum pump (think of a really power- ful vacuum cleaner) or a water aspirator. The aspirator is easy to use and readily available since all it requires is an adaptor and a water faucet. The aspirator is con- nected to a filter flask with a rubber vacuum hose and a Büchner funnel attached.
The rubber stopper on the bottom of the funnel seals the filtration flask so outside air is not pulled in from the top and the suction from the aspirator is concentrated on the contents of the Büchner funnel. As the water runs down through the faucet, it creates a suction in the side arm of the adaptor which pulls air from the connected filtration flask, creating a vacuum in the flask.
Fold in quarters
Open out into cone
Fold in half
Crease paper slightly
©Hayden-McNeil, LLC Figure 4.2. Gravity filtration.
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Filter paper
To aspirator on water faucet
Filtration flask
B üchner funnel
Filtrate
Separation Technique: Suction Filtration
©Hayden-McNeil, LLC
Figure 4.3. Separation technique: Suction filtration.
pH and Indicators pH is the change in the concentration of H^ ions in a solution. This change in concentration can be moni- tored in a number of different ways, including using a probe which constantly monitors pH or by using an indicator that changes color in a particular pH range. pH indicators are molecules that react to specific and small changes in pH by physically changing color with their protonation or deprotonation (addition or removal of a H^ ion). Many indicators are known, with phenolphthalein, methyl red, and bromophenol blue being some of the most commonly used. You will learn more about changes in pH and acid–base titrations in a later lab. However, you will be using a pH indicator in this experiment, methyl red.
Methyl red indicator, or simply methyl red, changes color when the pH changes between 4.4 and 6.4. Ini- tially with acidic solutions (those around pH 4 or less) methyl red will be red colored, and pink the closer to pH 4.4 the solution is. As the pH rises with the addition of base, the solution will turn yellow, as methyl red reacts to the change in pH. What is happening structurally to the methyl red when this occurs?
Materials and Procedures Limestone
3.00 M HCl
2—50 mL Erlenmeyer flask
150 mL Erlenmeyer flask
Methyl red indicator
Ammonium sulfate
2.00 M aqueous ammonia
Droppers
Glass stir rods
Oven
Procedure Carbonate in Limestone
- Obtain enough pieces of limestone so that you have two separate rock samples weighing ~0.70 grams to 0.80 grams and record the exact mass. See Chap- ter 3 for instructions on how to use an electronic balance. You will want to choose small pieces. If necessary, carefully use a hammer to further crush the limestone.
Acid form (red colored) Basic form (yellow colored)
HC
HC
C C
CH
CH
O
N +
N C
C
O–
H
C HC
CH
CH
CH
N
H 3 C
H 3 C
HC
HC
C C
CH
CH
O
N
N C
C
O –
C HC
CH
CH
CH
N
H 3 C
H 3 C
Figure 4.4. Methyl red.
E x p e r i m e n t 4 • Quantifying the Composition of Limestone
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- Put the watch glass into the oven to dry for 5 min- utes. Then remove the filter paper from the oven and scrape the sample onto the watch glass. Place the watch glass in the oven. Leave the sample in the oven to dry for at least 20 minutes.
- Remove the sample from the oven, allow it to cool for 5–10 minutes, and record its mass and any ob- servations. Check that you have your sample.
Data Analysis Make sure to show all of your calculations in your lab notebook as a record of how you completed your calcu- lations. Don’t forget to include your units and correct number of significant figures! Then, go onto Chem and report your results.
- Determine the mass of the carbon dioxide released.
- Determine the moles of carbonate in the limestone.
- Based upon your anion analysis, what is the per- cent composition by mass of calcium carbonate in limestone?
- Determine the mass of calcium sulfate that precipi- tated.
- How many moles of calcium carbonate are in the limestone based upon the cation analysis?
- Based upon your cation analysis, what is the percent composition of the calcium carbonate in limestone?
In your report, make sure you clearly identify the pur- pose of the experiment (are we testing a hypothesis or simply trying to accomplish something; what is the difference?). How will the purpose be met? How does the procedure fit what is stated in your introduction? Give specific support using your chemical understand- ing. Don’t forget to include any appropriate chemical reactions or mathematical equations, if appropriate, and explain them.
References
- Kentucky Geological Survey. Carbonates. http:// www.uky.edu/KGS/rocksmn/carbonates. htm#calcite (accessed May 8).
- Bliss, J. D.; Hayes, T. S.; Orris, G. J. Limestone—A Crucial and Versatile Industrial Mineral Commodity ; 2008–3089; Denver, CO, 2008 revised 2012.
- Kentucky Geological Survey. Limestone. http:// www.uky.edu/KGS/rocksmn/limestone. htm (accessed May 8).
Lab is adapted from Gregg Dieckmann and John Sib- ert’s “Percent Composition from Gravimetric Analysis: Calcium Carbonate in Texas Limestone.” An Atoms First Approach to the General Chemistry Laboratory. New York: McGraw-Hill. 2012.
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