Teacher Notes
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Teacher Notes![]() Gravimetric Analysis of Calcium and Hard WaterInquiry Lab Kit for AP® ChemistryMaterials Included In KitAntacid tablets, bottle of 20 Additional Materials RequiredWater, deionized or distilled, 40 mL* Prelab PreparationPrepare 100 mL each of six different “unknown” hard water samples for students to analyze in the guided-inquiry activity. See the calculations for the amount of 2 M CaCl2 stock solution to be diluted to prepare 100 mL of each sample. {13766_Preparation_Table_3}
Safety PrecautionsSodium carbonate is irritating to body tissues. Calcium chloride is moderately toxic by ingestion and the anhydrous generates a great deal of heat when dissolved in water. Avoid contact of all chemicals with eyes and skin. Antacid tablets used in the lab are considered laboratory chemicals and may not be removed from the lab. Do not taste or ingest any materials in the chemistry lab. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines. Please review current Safety Data Sheets for additional safety, handling and disposal information. DisposalPlease consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The leftover solid calcium chloride and sodium carbonate may be packaged for landfill disposal according to Flinn Suggested Disposal Method #26a. The leftover calcium chloride or sodium carbonate solutions may be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b. Lab Hints
Teacher Tips
Further ExtensionsAlignment to the Curriculum Framework for AP® Chemistry Enduring Understandings and Essential Knowledge Atoms are conserved in physical and chemical processes. (1E) 1E2: Conservation of atoms makes it possible to compute the masses of substances involved in physical and chemical processes. Chemical processes result in the formation of new substances, and the amount of these depends on the number and the types and masses of elements in the reactants, as well as the efficiency of the transformation. Matter can be described by its physical properties. The physical properties of a substance generally depend on the spacing between the particles (atoms, molecules, ions) that make up the substance and forces of attraction among them. (2A) 2A3: Solutions are homogenous mixtures in which the physical properties are dependent on the concentration of the solute and the strengths of all interactions among the particles of the solutes and solvent. Chemical changes are represented by a balanced chemical equation that identifies the ratios with which reactants react and products form. (3A) 3A1: A chemical change may be represented by a molecular, ionic, or net ionic equation. 3A2: Quantitative information can be derived from stoichiometric calculations that utilize the mole ratios from the balanced chemical equations. The role of stoichiometry in real-world applications is important to note, so that it does not seem to be simply an exercise done only by chemists. Learning Objectives 1.19 The student can design, and/or interpret data from, an experiment that uses gravimetric analysis to determine the concentration of an analyte in a solution. 2.10 The student can design and/or interpret the results of a separation experiment (filtration, paper chromatography, column chromatography, or distillation) in terms of the relative strength of interactions among and between the components. 3.2 The student can translate an observed chemical change into a balanced chemical equation and justify the choice of equation type (molecular, ionic, or net ionic) in terms of utility for the given circumstances. 3.3 The student is able to use stoichiometric calculations to predict the results of performing a reaction in the laboratory and/or to analyze deviations from the expected results. Science Practices 1.5 The student can re-express key elements of natural phenomena across multiple representations in the domain. 2.2 The student can apply mathematical routines to quantities that describe natural phenomena. 4.2 The student can design a plan for collecting data to answer a particular scientific question. 5.1 The student can analyze data to identify patterns or relationships. 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question. 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 7.1 The student can connect phenomena and models across spatial and temporal scales. 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understanding and/or big ideas. Answers to Prelab Questions
Sample DataSample Data for Introductory Activity {13766_Data_Table_4}
aPercent yield calculated based on the known molarity of CaCl2 provided to the students. {13766_Data_Equation_2}
where the factor 1/100 accounts for the fact that the sample analyzed was 100 times more concentrated than the original water sample. Conclusion Answers to QuestionsGuided-Inquiry Discussion Questions
Review Questions for AP® Chemistry
ReferencesAP® Chemistry Guided-Inquiry Experiments: Applying the Science Practices; The College Board: New York, NY, 2013. |
Student Pages
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Student Pages![]() Gravimetric Analysis of Calcium and Hard WaterIntroductionIn certain areas of the country, the presence of hard water poses significant problems in water supply systems. Various water softening techniques are used to remove the cations responsible for water hardness. This investigation involves the application of gravimetric analysis to test samples for the amount of water hardness and calcium ions. Concepts
BackgroundWater from natural sources may contain a number of dissolved substances. The amount and nature of these dissolved substances varies depending on the geography of the area and the journey the water has taken. As water travels through the ground or over the surface of the land, it can dissolve naturally occurring minerals. As minerals dissolve in the water, the compounds separate into their respective cations and anions. Common cations in water include Na+, Ca2+, Mg2+ and Fe3+ while the principal anions in water are Cl–, HCO3–, NO3– and SO42–. The main ions contributing to water hardness are Ca2+, Mg2+ and, to a lesser extent, Fe3+. Their presence makes it difficult for soaps to lather and also causes a “scum” to form. Equation 1 (where R is a long hydrocarbon chain) shows the precipitation reaction between alkyl sulfate anions in a typical soap with calcium ions in hard water. The main problem due to water hardness in industrial pipes or boilers is the buildup of solid CaCO3, which precipitates out and causes thick deposits to form in pipes and other appliances. {13766_Background_Equation_1}
There are many different ways to “soften” water. One of the most common ways to remove ions is by ion exchange. The ion exchange process uses a resin to replace some of the ions that cause hardness with ions that do not. Hardness is commonly measured in units of grains per gallon or milligrams per liter (also known as parts per million), and is classified by the U.S. Department of the Interior and the Water Quality Association as follows in Table 1:
{13766_Background_Table_1}
Although several ions contribute to water hardness, the units of mg/L or ppm are defined in terms of the equivalent mass (milligrams) of CaCO3 that would be present per liter of water. In this investigation, gravimetric analysis will be used to precipitate and isolate solid CaCO3 from water samples and determine water hardness. Many municipal water treatment plants use soda ash (sodium carbonate, Na2CO3) and lime (calcium hydroxide, Ca(OH)2) to chemically remove calcium and magnesium ions, respectively, from hard water.
Experiment OverviewThe purpose of this advanced inquiry lab is to investigate the suitability of gravimetric analysis for determining the amount of water hardness in the form of calcium carbonate, CaCO3, in various water samples. Six samples, representing a wide range of potential water hardness, from 50 ppm to 500 ppm, will be analyzed by various student groups as part of a cooperative class investigation to determine the accuracy and sensitivity of gravimetric analysis for water hardness testing. Note that all water samples have been concentrated by a factor of 100 for the purpose of quantitative analysis. The lab begins with an introductory activity to develop skill in the calculations and techniques of gravimetric analysis, in particular, quantitative transfer and vacuum or gravity filtration. The precipitation reaction involves preparing and combining solutions of Na2CO3 and CaCl2. The balanced chemical equation for this reaction predicts the amount of precipitate that will be formed. Careful isolation, drying and weighing of the precipitate will confirm the calculations and the percent yield. The procedure provides a model for guided-inquiry design of the cooperative class investigation described above. Antacid tablets are also provided as an opportunity for further inquiry—the use of gravimetric analysis to determine the amount of calcium in an over-the-counter medication. Materials
Antacid tablets (optional)
Calcium chloride, anhydrous, CaCl2, 2 g Sodium carbonate, anhydrous, Na2CO3, 2 g Sodium carbonate solution, Na2CO3, 0.5 M Water, deionized or distilled Balance, 0.001-g precision (shared) Beakers, 150-mL, 3 Drying oven (shared) Filter flask, 250-mL Filter paper, 3 Funnel, Büchner and rubber adapter Graduated cylinder, 50-mL Hard water samples, 20 mL, 2 Spatula Vacuum filtration apparatus setup Wash bottle Watch glasses, 2 Weighing dishes, 2 Prelab Questions
Safety PrecautionsSodium carbonate is irritating to body tissues. Anhydrous calcium chloride is moderately toxic by ingestion and generates a great deal of heat when dissolved in water. Avoid contact of all chemicals with eyes and skin. Antacid tablets used in the lab are considered laboratory chemicals and may not be removed from the lab. Do not taste or ingest any materials in the chemistry lab. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines. ProcedureIntroductory Activity Precipitation Reaction and Vacuum Filtration
Guided-Inquiry Design and Procedure
Analyze the Results Student Worksheet PDF |