Molarity and Dilutions

Transcription

1 Molarity and Dilutions Chemistry HS/ Unit: 10 Lesson: 02 Suggested Duration: 4 days Lesson Synopsis: Students will focus on the concept of the concentration of an aqueous solution as a measure of the amount of a solute dissolved in a specific amount of solvent, in this case, water. The calculation of the concentration of a solution and the preparation of diluted solutions from concentrated ones is integral to the study of the chemistry of chemical reactions in solution. TEKS: C.10 concepts. The student understands and can apply the factors that influence the behavior of solutions. The student is expected to: C.10C C.10D Calculate the concentration of solutions in units of molarity. Supporting Standard Use molarity to calculate the dilutions of solutions. Supporting Standard Scientific Process TEKS: C.2 Scientific processes. The student uses scientific methods to solve investigative questions. The student is expected to: C.2E C.2G C.2I Plan and implement investigative procedures, including asking questions, formulating testable hypotheses, and selecting equipment and technology, including graphing calculators, computers and probes, sufficient scientific glassware such as beakers, Erlenmeyer flasks, pipettes, graduated cylinders, volumetric flasks, safety goggles, and burettes, electronic balances, and an adequate supply of consumable chemicals. Express and manipulate chemical quantities using scientific conventions and mathematical procedures, including dimensional analysis, scientific notation, and significant figures. Communicate valid conclusions supported by the data through methods such as lab reports, labeled drawings, graphs, journals, summaries, oral reports, and technology-based reports. GETTING READY FOR INSTRUCTION Performance Indicator(s): Prepare a solution of a given solid in water at an assigned Molar concentration. Dilute the solution to a new assigned concentration. Write a summary explaining the procedure and the mathematical calculations used to create the initial and final solutions. (C.2E, C.2G, C.2I; C10C, C.10D) 1C; 3E; 5B Key Understandings and Guiding Questions: Solution concentrations can be calculated in mol/liter (M). What is molarity? How is solution concentration calculated? How is molarity used to calculate the dilutions of solutions? Vocabulary of Instruction: molarity Refer to section for materials. Handout: Making a 1.0 Molar Solution (1 per student) Teacher Handout: Making a 1.0 Molar Solution KEY Handout: Molarity Practice (1 per student) Teacher Resource: Molarity Practice KEY Handout: Solution Dilutions (1 per student) Teacher Resource: Solution Dilutions KEY 2012, TESCCC 05/22/13 page 1 of 9

2 Handout: Solution Practice Evaluation (1 per student) Teacher Resource: Solution Practice Evaluation KEY Teacher Resource: Performance Indicator Instructions KEY Advance Preparation: 1. Refer to MSDS sheets for all safety and disposal information of chemicals. 2. Prior to Day 1: Obtain/collect liquid commercial product containers with labels on (or separate) for concentration units. (e.g., hydrogen peroxide, isopropyl alcohol, vinegar, tincture of iodine, and saline solutions used for contact lenses and nasal washes) Pictures of product labels can be substituted for commercial products. Obtain plastic BBs (or beads) of two colors (such as red and green) and plastic cups. If plastic BBs are not available, other materials can be used as long as they can be differentiated in the model to determine concentration. Note: One color BB will represent the solvent and the other the solute. Prepare cups of the same or varying model concentrations of one color in the other color. One example would be 90 red and 10 green BBs, which would then model a 10% by particle solution. 3. Prior to Day 2: Organize and collect necessary materials and equipment for making 1.0 M NaCl solution for the investigation. Be sure to have enough NaCl available for multiple student groups throughout the day. Determine a system for student groups to label and store their solutions for Day 3 - making dilutions. If you do not have multiple volumetric flasks, prepare to demonstrate their use in this investigation. Have students make observations and summarize ways the volumetric flask and Erlenmeyer flask are similar and different (appearance and use). 4. Prior to Day 3, organize and collect necessary materials and equipment for the Making Solution Dilutions investigation. 5. Prior to Day 4: Determine different molarities of solutions to distribute to student groups. Prepare a set of lab stations with the materials specified, as available. Have different solutions and dilutions for each group. Prepare cards with different solutions cited. Create enough to avoid having duplicate solutions to ensure mastery of the skill. Consider the amount of solutes/solvents available when determining molarities. 6. Prepare attachment(s) as necessary. Background Information: This unit bundles student expectations related to solutions such as the properties of water, types of solutions, concentration, and solubility. Prior to this unit, students have been introduced to solutions in both elementary and middle school. During this unit, students begin to develop a greater understanding of solubility and its relationship to the structure of water and properties of solutions and learn how to calculate the concentration of a solution. This is the last time these concepts will be directly addressed before the STAAR assessment. STAAR Note: Factors influencing the behavior of solutions will be tested as Readiness Standard under Reporting Category 5: Solutions. The amount of the solute that is present in a given amount of the solvent is referred to as the concentration of the solute in the solvent or simply concentration. A solution that has been made accurately, to a known concentration, is called a standard solution. Concentration may be expressed in several different ways, depending upon the use of the solution. Scientists usually use molarity (M - moles of solute per liter of solvent) when expressing concentration of solutions. Commercial products often use percentage by volume (3% hydrogen peroxide (v/v) is three volumes of hydrogen peroxide in 100 volumes of solution) or percentage by mass (10% NaCl solution is 10 grams of NaCl in 100 grams of solution). Another less-used unit is molality, a measure of the moles of solute per kilogram or 1000 grams of solvent. 2011, TESCCC 01/19/12 page 2 of 9

3 When a solid dissolves in a liquid, the forces that hold the solid particles (molecules, atoms, or ions) together must be less than the force that the liquid exerts on the particles to pull them apart. In order for the solid to stay in solution, the attraction between the molecules and the solvent must be greater than the forces which bond the molecules together. Generally, ions and polar molecules are attracted by polar solvents and, therefore, dissolve in polar solvents. Non-polar molecules dissolve more readily in solvents that are non-polar. This is known as the Like Dissolves Like rule. As included in the STAAR Chemistry Reference 1. The molarity (M) of a solution is found using: moles of solute M = Liter of solution 2. Dilutions may be computed using: V 1M 1 = V 2M 2, where V is solution volume (L or ml) and M is molarity (mol/l). The mole concept, dimensional analysis, unit analysis, and basic mathematical formula manipulation are used to determine mass and volume in preparing solutions of known molarity. Chemical quantities can be expressed using scientific notation and significant figures. GETTING READY FOR INSTRUCTION SUPPLEMENTAL PLANNING DOCUMENT Instructors are encouraged to supplement and substitute resources, materials, and activities to differentiate instruction to address the needs of learners. The Exemplar Lessons are one approach to teaching and reaching the Performance Indicators and Specificity in the Instructional Focus Document for this unit. Instructors are encouraged to create original lessons using the Content Creator in the Tools Tab located at the top of the page. All originally authored lessons can be saved in the My CSCOPE Tab within the My Content area. INSTRUCTIONAL PROCEDURES Instructional Procedures ENGAGE Solution Concentrations NOTE: 1 Day = 50 minutes Suggested Day , TESCCC 01/19/12 page 3 of 9

4 1. Divide the class into groups of 2 4 students. 2. Provide each group with several commercial products or product labels that show some type of concentration units on the label (see Advance Preparation). 3. Project a class chart of the commercial products. Leave room for showing solute, solvent, and concentrations from the commercial product labels. 4. Work with students to identify and list the solute, solvent, and concentration of the commercial product solutions. 5. Facilitate a class discussion in which students reflect on the different ways that concentrations are expressed. Inform students that during the upcoming lesson, they will learn how to prepare solutions of known concentrations. 6. Discussion of the different ways that concentrations are expressed might include the following: Most commercial products will be percent solutions. Biologists often use percentage solutions when comparing solution concentrations. Percentage solutions can be in the form of percentage by mass or percentage by volume. Percentage solutions use the mass or volume of the solute in relation to the mass or volume of the solution (solute + solvent). commercial products or product labels (see Advance Preparation, several per Safety Note: Refer to MSDS sheets and labels for safe handling and any needed disposal of chemicals. Student Notebooks: Students create a chart showing solute, solvent, and concentrations from commercial product labels. Then, students summarize the class discussion. 7. Instruct students to recreate the class chart in their science notebooks, recording the information discussed, and then to summarize the main ideas from the discussion. As time allows, ask students to share with a partner and then to revise and add to their summaries. EXPLORE I Modeling Solution Concentration 1. Divide the class into groups of 2 4 students. Suggested Day 1 (continued) 2. Distribute materials to each group, including a cup with a model of a solution using BBs (see Advance Preparation) and two empty cups. Explain that the cup of mixed BBs represents an aqueous (water) solution. Pose the following questions: What is the solvent in your model solution? (The color of most of the BBs) What is the solute in your model solution? (The color of the other BBs) 3. Instruct students to use the empty cups to help find the solute percentage concentration of the solution by counting the BB particles and then to record their findings in their science notebooks. Instruct them to make labeled drawings of each of the models to include solvent and solute. 4. Using a cooperative grouping strategy, ask students to share their answers with the class. Instruct students to discuss their measurements, calculations, and any problems or questions they have about the concepts. 5. To conclude, instruct students to mix their BBs back into their original cup. Inform students of how you made the solutions all the same or different. clear cup with a mixture of two colors of plastic BBs (see Advance Preparation, 1 per clear cups (2 empty per Safety Note: Students should handle BBs with care so that they do not fall on the floor and cause a slipping hazard. Instructional Notes: See the Advance Preparation section. Make solutions cups with the same or several different concentrations depending on how you want to guide discussion. One example would be 90 red and , TESCCC 01/19/12 page 4 of 9

5 6. Explain that chemists express solution concentration in the form of molarity (M) - number of moles of solute per liter of solution. Discuss the following: What is molarity? A unit of concentration, symbolized by "M". It is the ratio of the number of moles of solute to the volume of solution (in liters). How is solution concentration calculated? (Moles of the solute in the solution divided by the total volume of the solution) How many solute particles would there be in 1 L of a 1 M solution? 6.02 x green BBs, which would then model a 10% by particle solution. Notebooks: Students record findings from the modeling activity in their notebooks. EXPLORE/EXPLAIN II Making a 1.0 M Solution Suggested Day 2 1. Divide the class into groups of four. Inform them that each group will make one liter of a 1.0M solution of sodium chloride. 2. Demonstrate the use of a 1000 ml volumetric flask in the preparation of solutions. Explain why it is an accurate and expensive piece of glassware. Demonstrate the use of a pipette for the final addition of water for precision. 3. Stress laboratory safety as well as careful laboratory skills. 4. Remind students they will write detailed procedures prior to beginning any work. 5. Explain the disposal of unused NaCl and general clean up. NOTE: Inform students you will be collecting the 1.0 M NaCl solutions to use later for a dilution investigation. 6. Distribute the Handout: Making a 1.0 Molar Solution, and ask students to affix it in their notebooks. 7. Instruct students to work through step 3 and then to let you know they are ready to make their solution (step 5). sodium chloride (about grams per group in a small, plastic cup, per spatulas (1 2 per pipette (1 per balance (1 per massing paper (1 2 sheets per volumetric flask (1000 ml, 1 per deionized or distilled water (per sealable container (large, for NaCl solutions, 1 per safety goggles (1 pair per student) apron (1 per student) glue or tape (per 8. Monitor student progress, and address any difficulties. 9. Although each student has an individual handout, approve each group s procedure before they begin the preparation of their solution. 10. When students are finished making their solutions, instruct them to rinse the volumetric flasks and clean up their area. Students should follow rules regarding chemical disposal. 11. Note: Collect the 1.0 M NaCl solutions because they will be used for tomorrow s lesson with solution dilutions. 12. Instruct students to write the procedures for the preparation of 1.0 M and 2.0 M solutions of sodium carbonate (handout step 7 and 8) in their science notebooks. Handout: Making a 1.0 Molar Solution (1 per student) Teacher Resource: Making a 1.0 Molar Solution KEY Handout: Molarity Practice (1 per student) Teacher Resource: Molarity Practice KEY Safety Notes: Everyone should be wearing safety goggles throughout the activity. 2011, TESCCC 01/19/12 page 5 of 9

6 13. Provide 2 3 minutes for students to share orally with a partner their procedures for making the solutions. Instruct students to use academic vocabulary words in their descriptions such as molarity, volumetric flasks, solute, and solvent. Ask them to also explain how they used dimensional analysis in the calculations. 14. While students are sharing orally, walk around the room to clarify and address and problems with the procedures. 15. Discuss the sodium carbonate procedures with the class, and then review the preparation of any amount of 1.0 M solution. 16. Distribute the Handout: Molarity Practice. Model for students how to work the calculations for solutions of different volumes and different molarities. It may be helpful to work some practice problems together. 17. Instruct students to open their STAAR Chemistry Reference Materials and locate the section labeled, Solutions. As the two new solutions calculations and formulas are introduced, draw students back into this document for reference and to complete calculations with molarity and concentrations. 18. Assign the problems to be worked in groups or as homework. Check answers as appropriate. Volumetric flasks are expensive and should be handled carefully. Instructional Notes: A 1000 ml volumetric flask is optimum for making this solution as it is a required type of glassware according to TEKS C.2E. Adapt the instructions to your glassware. You may want to or need to use smaller volumetric flasks. If so, adjust the procedure as necessary. Since this may be the first time students have used a volumetric flask, take a few minutes to have students compare and contrast the appearance and uses of volumetric and Erlenmeyer flasks. Solutions should always be prepared with deionized or distilled water. STAAR Notes: Instruct students to open their STAAR Chemistry Reference Materials and locate the section labeled, Solutions. As the two new solutions calculations and formulas are introduced, draw students back into this document for reference and to complete calculations with molarity and concentrations. Notebooks: Students write all procedures, observations, data, and calculations in their science notebooks. Additionally, students should work sample problems in their science notebooks. EXPLORE/EXPLAIN III Solution Dilutions Suggested Day 3 1. Divide the class into groups of four. 2. Instruct each group they are to prepare 100 ml of a 0.25 M NaCl solution using the materials and equipment you have prepared (see Advance Preparation). 3. Challenge students to design a procedure to answer the challenge. Remind them of the two prior exercises if additional direction is needed. 4. Inform students they must write out their procedures in their notebooks and sodium chloride (about 2 grams in small plastic cups per spatulas (1 2 per pipette (1 per balance (1 per massing paper (1 2 sheets per 2011, TESCCC 01/19/12 page 6 of 9

7 that you must approve them before they make their solutions. 5. Review and amend their procedures as needed. 6. Demonstrate proper use of any equipment as needed. 7. Monitor students as they make their 0.25 M solutions. 8. Instruct students to set their solutions aside and return to their seats. 9. Pose the following question: What is a dilution? Accept appropriate answers. 10. Explain that dilutions are made from a solution of known concentration. Point out that a dilution cannot be more concentrated than the original solution. 11. Introduce the formula V 1 M 1 = V 2 M 2, where: M 1 is the molarity of the solution of known concentration. M 2 is the molarity of the diluted solution. V 1 is the volume of the solution of known concentration. V 2 is the volume of the diluted solution. 12. Model examples for the class using the dilution formula. Work several examples as a class. Show that the formula works with volumes measured in either liters (L) or milliliters (ml). 13. Instruct students to make 100 ml of a 0.25 M NaCl solution using the stock solution, 1.0 M NaCl that they produced previously. Challenge the students to design a procedure in order to answer the question. 14. Remind them to write out their procedures for your approval before they prepare their solutions. 15. Direct students to clean up the volumetric flasks and their area when they have completed making their solutions. Students should follow your rules regarding chemical disposal. 16. Discuss the following: How is molarity used to calculate the dilutions of solutions? Dilutions are computed using: V 1M 1 = V 2M 2, where V is solution volume and M is molarity. 19. Distribute a copy of the Handout: Solution Dilutions to each student. Assist students in working through the dilutions with their lab groups. 20. Monitor groups as needed, addressing any issues. volumetric flask (100 ml, 1 per volumetric flask (1000 ml, 1 per beaker (250 ml, 1 per Erlenmeyer flask (any volume, 1 per container of 1.0 M NaCl solution (previously produced, per distilled or deionized water (per graduated cylinder (10 or 25 ml, 1 per safety goggles (1 pair per student) apron (1 per student) Handout: Solution Dilutions (1 per student) Teacher Resource: Solution Dilutions KEY Safety Notes: Students should be wearing safety goggles and aprons throughout the activity. Refer to the MSDS sheets for each chemical for safety and disposal instructions. Instructional Notes: The 100 ml volumetric flask is optimum for making this volume of solution. If you have a 25 ml pipette, demonstrate and explain its use. STAAR Note: The STAAR Chemistry Reference Materials include formulas for calculation of Molarity and Concentration. Notebooks: Students record all procedures, observations, data, and calculations in their science notebooks. 2011, TESCCC 01/19/12 page 7 of 9

8 ELABORATE Solution Practice 1. Distribute a copy of the Handout: Solution Practice Evaluation to each student. 2. Allow students time to read the handout, and address any questions the students may have regarding the instructions. 3. Instruct students to work through the problems. This may be assigned as homework if necessary. 4. Explain the Performance Indicator activity you have set up for the next day. Suggested Day 3 (continued) Handout: Solution Practice Evaluation (1 per student) Teacher Resource: Solution Practice Evaluation KEY Instructional Note: This activity is provided as a practice activity for students and can be assigned as homework. EVALUATE Performance Indicator Suggested Day 4 Performance Indicator Prepare a solution of a given solid in water at an assigned molar concentration. Dilute the solution to a new assigned concentration. Write a summary explaining the procedure and the mathematical calculations used to create the initial and final solutions. (C.2E, C.2G, C.2I; C10C, C.10D) 1C; 3E; 5B 1. Refer to the Teacher Resource: Performance Indicator Instructions KEY for information on administering the assessment. soluble salts (such as potassium nitrate, potassium sulfate, sodium bicarbonate, per balance (1 per massing paper (1 2 sheets per cups or beakers (per pipette (1 per volumetric flask (100 ml, 1 per beaker (see Instructional Notes, 250 ml, 1 per Erlenmeyer flask (see Instructional Notes, any volume, 1 per beaker (see Instructional Notes, 500 ml, 1 per volumetric flask (see Instructional Note, 1000 ml, 1 per graduated cylinder (10 or 25 ml, 1 per distilled or deionized water (per safety goggles (1 pair per student) apron (1 per student) Teacher Resource: Performance Indicator Instructions KEY Check for Understanding: This activity provides an opportunity for authentic assessment of student laboratory skills as well as student calculations. 2011, TESCCC 01/19/12 page 8 of 9

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