What's In My Water???

Lesson Content

• Lesson Plan Template:

  General Lesson Plan

• Learning Objectives: What will students know and be able to do as a result of this lesson?

  
  
  • The student will be able to determine the precipitate of a reaction between two aqueous solutions.
  • The student can use the mass of a precipitate in a chemical reaction to perform stoichiometric calculations to determine the amount of a substance (or concentration) of an analyte in a solution.
  • The student is able to design an experiment that uses a separation technique such as filtration in order to answer a particular scientific question.

• Prior Knowledge: What prior knowledge should students have for this lesson?

  
  
  • Students should have a deep understanding of the following topics prior to performing this activity:
  • Polyatomic ions such as sulfate, SO4 2-
  • Balancing chemical reactions
  • Double replacement/Precipitation Reactions (Net Ionic Reactions)
  • Predicting precipitates in precipitation reactions
  • Stoichiometric calculations
  • Filtration of precipitates – combining two aqueous solutions, filtering a precipitate, washing a precipitate, and drying a precipitate
  • Pipetting techniques or measuring volume of solutions
  • Using a digital balance to determine mass in grams
  • Concentration measurements or calculation of ratios such as mg/mL or moles/L

• Guiding Questions: What are the guiding questions for this lesson?

  
  
  • What is the balanced chemical reaction that will be completed in this laboratory?
  • What is the balanced net ionic chemical reaction for this experiment?
  • What volume of the water sample aliquot will be used for this experiment?
  • What precipitate will form as a result of this chemical reaction?
  • How many grams of precipitate were collected?
  • How many moles of precipitate were collected?
  • How many moles and subsequently grams of sulfate were present in the water sample aliquot?
  • Does the sample meet the criteria or fail the criteria for maximum amount of sulfate in a water solution?

• Introduction: How will the teacher inform students of the intent of the lesson? How will students understand or develop an investigable question?

  
  

  Introductory Lesson

  Teacher will introduce this topic of investigation through the everyday example of "hard water."

  Many times we find it difficult to lather soap in the shower or find soap scum in the shower due to hard water. This hard water contains metal ions such as Ca2+ that form precipitates or an insoluble compound. If there is hard water forming precipitates then soap is not as effective and even it can affect our laundry and hair.

  We often use water softeners to replace the calcium ions, Ca2+, that precipitate with soluble sodium ions, Na+. Some water softeners actually precipitate out the calcium ions prior to entering in our water heaters. We actually measure the amount or concentration of this analyte of Ca2+ (measure the hardness of our water) in a process called gravimetric analysis. Basically we are able to take an aliquot of this water from the home, precipitate out an ionic compound that contains the Ca2+ ion, filter it, wash it, dry it, and finally measure the mass of the precipitate. We are able to use stoichiometric calculations in order to determine how many moles per liter or milligrams per liter (the concentration) and compare it to a standard for criteria. If a home has 120-150 mg/L as CaCO3, it is recommended to have a water softener installed. A common reaction used for gravimetric analysis (or to precipitate out a specific ion in solution) of hard water is:

  Ca2+ (aq) + Na2CO3 (aq) 2 Na+ (aq) + CaCO3 (s)

  Introductory Laboratory

  Teachers may utilize an introductory experiment of gravimetric analysis using the below procedure:

  1. Weigh about 1 gram of sodium carbonate, Na2CO3, in a clean, dry beaker.
  2. Weigh about 1 gram of calcium chloride, CaCl2, in a clean, dry beaker.
  3. Add approximately 20 mL of distilled water. Stir until each solid is dissolved.
  4. Warm up both solutions until moderately warm (approximately 40-60°C). Add the sodium carbonate solution into the calcium chloride solution very slowly forming a precipitate. The slower the solution is added and stirring between each addition will result in a better precipitate (and lessen the chance of an emulsion forming).
  5. Weigh a piece of filter paper and set up the filtration apparatus.
  6. Insert the filter paper into the funnel land wet with deionized water.
  7. Pour the contents of the beaker slowly into the funnel and perform wash and rinse on the empty beaker and the precipitate on the filter paper.
  8. Weigh a watch glass and carefully remove the filter paper with the precipitate and set it on the watch glass.
  9. Dry the precipitate in a drying oven (approximately 110-120°C) for 15-20 minutes.
  10. Remove the watch glass, break up the precipitate carefully and reheat for an extra 5 minutes.
  11. Remove the watch glass, cool, and weigh.
  12. Dry the precipitate for an extra 5 minutes, remove, cool, and weigh. Continue this until a constant mass is measured to ensure the precipitate is fully dry.

  Sample Student Data:

  Mass of Na2CO3 = 1.032 g

  Mass of CaCl2 = 1.025 g

  Mass of filter paper = 0.323 g

  Mass of watch glass = 28.344 g

  Mass of watch glass, filter paper, and dried precipitate (First Dry) = 29.532 g

  Mass of watch glass, filter paper, and dried precipitate (Second Dry) = 29.527 g

  Student Questions:

  1. Write the balanced chemical reaction (including states of matter) for this experiment.

  Na2CO3 (aq) + CaCl2 (aq) CaCO3 (s) + 2 NaCl (aq)

  2. Write the net ionic chemical reaction for this experiment.

  Ca2+ (aq) + CO3 2- (aq) → CaCO3 (s)

  3. Using the sodium carbonate, predict the mass of calcium carbonate that will be produced.

  1.032 g Na2CO3 / 105.99 g/mol x (1 mole CaCO3/1 mole Na2CO3) x 100.09 g/mol = 0.975 g CaCO3

  4. Using the calcium chloride, predict the mass of calcium carbonate that will be produced.

  1.025 g CaCl2 / 110.98 g/mol x (1 mole CaCO3/1 mole CaCl2) x 100.09 g/mol = 0.924 g CaCO3

  5. Which substance was the limiting reactant, sodium carbonate or calcium chloride?

  Calcium chloride since this substance was total used in the reaction to only produce 0.924 g of CaCO3

  6. Using the final mass, determine the number of grams of precipitate, CaCO3, that is produced.

  29.527 g of watch glass, filter paper, and precipitate – 28.344 g watch glass – 0.323 g filter paper = 0.860 g CaCO3

  7. What is the percentage yield of precipitate, CaCO3?

  % Yield = 0.860 g / 0.924 g x 100% = 93.07 % yield

  Introductory Virtual Laboratory

  After an understanding of the procedure of a gravimetric analysis laboratory, students can also perform a virtual laboratory of gravimetric analysis performing stoichiometric calculations using the mass of the precipitate and determining the moles, mass, and concentration of the limiting reactant.

  Virtual Laboratory can be found at: http://www.chemvlab.org/activities/activity.php?id=3

  This virtual laboratory will assist students in recognizing the ions that will precipitate and the ions that will simply be spectator ions. It will help with students ability to balance chemical reactions, calculating moles and concentration of a substance that was initially studied.

• Investigate: What will the teacher do to give students an opportunity to develop, try, revise, and implement their own methods to gather data?

  
  

  Students in groups of 2 will be given two small aliquots of a water solution (20 mL) and asked the following investigatory question:

  "Does the amount of sodium sulfate in the water meet the maximum criteria of 8 mg/mL as set by safety regulations?"

  Students will determine the goal of the investigation and develop their own laboratory procedure using gravimetric analysis to answer the question with a claim backed by evidence. Students will be given all materials set forth below including a classroom stock solutions of barium nitrate, Ba(NO3)2, sodium nitrate, NaNO3, and magnesium sulfate, MgSO4, to possibly use (they are only to pick on of these stock solutions to use).

  Teacher procedure for preparation of barium nitrate, Ba(NO3)2: 65.343 grams of barium nitrate dissolved in 500. mL of water produces a 0.500 solution of barium nitrate ensuring the sulfate ions will be the limiting reagent.

  Sodium nitrate and magnesium sulfate solutions can be prepared in any concentration since these two stock solutions will not precipitate with sodium sulfate and are therefore distractors for students to understand how to pick a correct solution that will precipitate.

  Other materials needed for this laboratory are as follows: pipets for measuring 10 mL or 10 mL quantitative graduated cylinders, clean beakers, heater, filter paper, funnels, electronic balance that measures to at least 0.001 g, watch glasses, heater, and other common laboratory equipment.

  Teacher will prepare two aliquot solutions for each group prior to class as follows:

  Example of a sample prep below the maximum criteria: 0.080 grams in 20 mL of distilled water = 4 mg/mL

  Example of a sample prep above the maximum criteria: 0.240 grams in 20 mL of distilled water = 12 g/mL

  Teacher can have student groups develop their plan of investigation first and either have it approved or peer reviewed. Possible questions to guide students during this inquiry laboratory investigation are:

  1. What is the balanced chemical reaction that will be completed in this laboratory?

  Na2SO4 (aq) + Ba(NO3)2 (aq) → BaSO4 (s) + 2 NaNO3

  2. Which stock solution will you choose to use and why?

  Ba(NO3)2 since the barium ions will precipitate with the sulfate ions. Sodium nitrate will not precipitate nor will magnesium sulfate.

  3. What volume of the unknown water sample aliquot you have been given will be used for this experiment?

  10.0 mL (so that a second trial could be done)

  4. What precipitate will form as a result of this chemical reaction?

  BaSO4, barium sulfate

  5. What data are you looking to measure in this laboratory?

  Volume of sodium sulfate solution, Mass of filter paper, Mass of watch glass, Mass of watch filter paper, watch glass, and precipitate after heating

• Analyze: How will the teacher help students determine a way to represent, analyze, and interpret the data they collect?

  
  

  Student groups will use inquiry laboratory rubric (attached) to help develop and organize their laboratory report (exemplary student laboratory report attached). Student groups will work as partners to work through the stoichiometric calculations to determine the concentration of sodium sulfate in the original solution.

  Teacher may guide students through use of socratic questioning to help them understand their data. Possible questions that teacher may pose are below:

  1. What was the mass of the precipitate in the reaction?

  Example data = 0.188 grams

  2. What is the molar mass of the precipitate in the reaction?

  233.39 grams/mol

  3. What are the number of moles of precipitate in the reaction?

  Example data = 0.188 grams / (233.39 g/mol) = 0.000806 moles

  4. What are the number of moles of sodium sulfate in the original solution?

  Example data = (1:1 ratio of BaSO4 and Na2SO4) = 0.000806 moles

  5. What are the number of grams of sodium sulfate in the original solution?

  Example data = 0.000806 moles x (142.04 g/mol) = 0.114 grams

  6. What is the concentration in mg/mL of sodium sulfate in the original solution?

  Example data = 0.114 grams x (1000 grams/1 mg) / 10 mL = 11.45 mg/mL

  7. Does the sample pass or fail the safety maximum criteria of 8 mg/mL?

  No, the sample fails and is over the 8 mg/mL criteria.

  Students will write an inquiry laboratory report and have it peer reviewed for content and calculations before turning in the final draft.

• Closure: What will the teacher do to bring the lesson to a close? How will the students make sense of the investigation?

  
  

  Students will have their inquiry laboratory report grading according to the inquiry rubric.

  Students will also do a post-lab assessment by answering the following questions.

  1. Excess Ba(NO3)2 (aq) is added to a 35.54 mL sample of Na2SO4 (aq).

  (a) What is the formula for the precipitate?

  BaSO4 (s)

  (b)If 4.553 grams of precipitate were formed, what is the concentration of Molarity (moles/Liter) of the original sodium sulfate solution?

  4.553 grams BaSO4 / (233.43 g/mol) = 0.01950 moles BaSO4

  (1:1 ratio of BaSO4 and Na2SO4) = 0.01950 moles of Na2SO4

  0.01950 moles of Na2SO4 / 0.03554 L = 0.5487 moles/L of Na2SO4

  2. A 5.000 gram mixture containing potassium nitrate and sodium chloride. Excess lead (II) nitrate solution, Pb(NO3)2, is added to make a precipitate of 0.950 grams.

  (a) What is the balanced chemical reaction to produce a precipitate? Label the precipitate as solid (s).

  Pb(NO3)2 (aq) + 2 NaCl (aq) → PbCl2 (s) + 2 NaNO3 (aq)

  (b) How many grams of NaCl was present in the original solution?

  2.503 grams PbCl2 / (278.10 g/mol) = 0.00900 moles PbCl2

  (1 mole PbCl2 : 2 moles NaCl) = 0.0180 moles NaCl

  0.0180 moles NaCl x (58.44 g/mol) = 1.052 grams NaCl

  (c) What is the percent by mass of sodium chloride in the original mixture?

  1.052 grams NaCl / 5.000 grams mixture x 100% = 21.04% by mass

• Summative Assessment

  
  

  Students will turn in a peer reviewed final draft of the inquiry laboratory report demonstrating sufficient background knowledge of gravimetric analysis, the methodology of the laboratory design including a detailed procedure and list of quantitative data collected during the experiment, correct analysis of data using stoichiometric calculations, and an answer to the claim using evidence.

  Students will also perform a post-laboratory assessment with two free-response questions where students can demonstrate their understanding and knowledge of precipitation reactions and stoichiometry to determine the amount of an original substance.

• Formative Assessment

  
  

  During the introductory lesson, teacher can use a demonstration of hard water precipitation as opposed to "soft water" or distilled water and question students observations of a precipitate.

  During the introductory laboratory, teacher will continually ask students to explain the purpose for performing steps in the laboratory procedure. Examples would be to assess student understanding of why they are washing their precipitate, why they are doing multiple dries of their precipitate, what would happen to their results if their precipitate was not totally dry, how a precipitate will help them determine what was in the original solution. These assessment questions will help laboratory groups come to a greater depth of understanding the design of a filtration experiment as well as the purpose of the stoichiometry behind the laboratory. It will also gauge student understanding of the laboratory design.

  During the introductory virtual laboratory, teacher will use the conclusion assessment questions (Screen 23 of 23) at the end of the virtual laboratory to assess student understanding. These questions include a student understanding that the mass of the precipitate is dependent upon the moles of the limiting reactant, student knowledge of how knowing the amount of the precipitate help us calculate the amount of a substance in the original solution, and student understanding of what happens to ions throughout a reaction.

  During the actual inquiry investigation, formative assessment will be conducted as students show the teacher their proposed design of a laboratory, guided investigation questions found in the Investigate section, and rough drafts of their inquiry laboratory report.

• Feedback to Students

  
  

  During the introductory lesson and laboratory, teacher will help to clarify understanding of both stoichiometric calculations and laboratory procedure through Socratic questioning (asking questions intended to encourage discussion and critical thinking) and student explanations.

  During the introductory virtual laboratory, students are given instantaneous feedback by the simulation through error messages and "Hint" screens. Teacher can review and discuss their final conclusion answers in a large class discussion following the completion of the virtual laboratory.

  During the inquiry laboratory, teachers can approve and/or give appropriate feedback to student design of the laboratory procedure. The peer review and teacher review of rough drafts of the inquiry laboratory report will assist in a constructive and focused feedback to students. Peer reviewers will help to give feedback by explaining how the author might improve their report section by section.