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The Gravimetric Analysis of Lead in Contaminated Water Mark as Favorite (26 Favorites)

LAB in Solubility, Concentration, Molarity, Stoichiometry, Solubility Rules, Reactions & Stoichiometry, Solutions, Chemical Technical Professionals. Last updated May 02, 2023.

Summary

In this lab, students will perform a gravimetric analysis of a simulated water sample contaminated with “lead”. Using their knowledge of solubility and chemical reactions they will precipitate the “lead” from the water sample. Then from the data collected, they will calculate the concentration of “lead” in their samples and compare that value to those found in water samples from the Flint, Michigan water crisis.

Grade Level

High School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

  • HS-PS2-6. Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
  • HS-ETS1-3: Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts.
  • Scientific and Engineering Practices:
    • Developing and Using Models
    • Analyzing and Interpreting Data
    • Constructing Explanations and Designing Solutions

Objectives

By the end of this lab, students should be able to:

  • Use solubility rules to determine if the product in a chemical reaction will form a precipitate.
  • Isolate a precipitate and determine its mass through a laboratory procedure.
  • Use collected laboratory data to complete stoichiometric calculations.

Chemistry Topics

This lab supports students’ understanding of:

  • Solutions
  • Solubility
  • Precipitate
  • Solubility Rules
  • Molarity
  • Concentration
  • Chemical Reactions
  • Stoichiometry

Time

Teacher Preparation: 20 minutes (solution preparation)

Lesson: 90 minutes

Materials (per group)

Safety

  • Teachers should consult the information provided in the associated SDS for each of the solutions (each linked in Materials section) used in the lab portion of the lesson.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
  • Always wear safety goggles when handling chemicals in the lab.
  • Students should wash their hands thoroughly before leaving the lab and at any time if skin comes in contact with solutions.

Teacher Notes

  • To learn more about this lab, read the associated article, Exploring Social Justice Through a Chemistry Lens, featured in the May 2023 issue of Chemistry Solutions.
  • This lab is intended to be used as the fourth part of a semester-long project. Unlike a typical project that is confined to a particular unit or topic, the Flint Water Crisis is used as a focal point of the larger project and is revisited as needed throughout the semester. A variety of chemistry topics and activities are connected to it during this time.
  • Teachers can use this particular activity independent of the larger project or review the related article for more information about the activities that precede and follow this one. In the five-part project, students apply concepts including solution chemistry, stoichiometry, and electrochemistry to the ongoing water crisis in Flint, Michigan. In the process, they also learn about both the science and the societal impact of this issue.
  • It is suggested that students complete the pre-lab questions as a homework assignment in advance of the lab day.
  • Students should complete the lab activity in small groups.
  • Through the pre-lab questions students should determine that they need to use the sodium carbonate solution, Na2CO3(aq), in order to precipitate the “lead” from the contaminated water sample. Teachers do not need to prepare the other solutions offered in the pre-lab section, since students will not use them.
  • In this lab the contaminate water sample is a solution of calcium chloride, CaCl2, not lead, due to the toxicity of lead. Teachers should prepare 0.5M calcium chloride solution in advance, enough for each group to use ~25 ml. Note that combining calcium chloride with water will produce heat.
  • Students will react the contaminated water with 0.5M sodium carbonate solution, NaCO3(aq), producing CaCO3 as a precipitate. Note that students should perform all calculations as if the solid produced is lead (II) carbonate PbCO3 for the simulated purposes of this lab.
  • Students should be proficient in using a solubility table to predict products.
  • An Answer Key document has been provided for teacher reference.

For the Student

Lesson

Introduction

In this lab, you will work to quantitatively determine the Pb2+ content of a water sample. In order to isolate the Pb2+, we will add a solution that contains ions that will bind with the Pb2+ to form a solid precipitate. Then, this solid can be filtered out, washed, dried and weighed to determine the mass of Pb2+ present in the original water sample. Analytical methods like this one, where the substance of interest is isolated in a precipitate and its mass is determined are called gravimetric analyses. Written in step form, the general procedure is as follows:

  1. Conduct a precipitation reaction to convert the Pb2+ in the water into a solid product.
  2. Filter out the solid product.
  3. Rinse and dry the product.
  4. Determine the mass of the product.

*Note: For the purposes of this lab, due to the toxicity of lead, we will be using a different metal ion in its place. We will proceed as if we are testing for lead but the actual ion used is safer and can be disposed of more easily.

Pre-Lab Questions

  1. You have access to the following solutions: NaNO3(aq), KI(aq) and Na2CO3(aq). Based on what you know about solubility, which solution should you use to isolate the Pb2+ from the contaminated water? Explain your choice.

  1. Write a balanced net ionic equation for the reaction taking place to form the solid lead compound.
  1. Identify the spectator ion in this reaction.
  1. What is the solid product that you will collect via filtration?
  1. How will you know when you have added enough solution to precipitate out all of the Pb2+?

  1. Draw two particle diagrams representing the solutions and the hard water before mixing and a third particle diagram, representing the solution and the water after mixing.
  1. Watch this short video on filtration (6:00 min) and answering the following questions:
    1. Why do we need to weigh the filter paper before and after filtration?
    1. What is the purpose of “washing” the precipitate with water?
    1. When you are done, you want to dry the product in the oven until the mass stops changing. This is called “heating to constant mass”. Why is this necessary?

Materials

  • Contaminated water sample
  • One of the following selected solutions:
    • NaNO3(aq), KI(aq) or Na2CO3(aq)
  • Graduated Cylinder
  • Beaker
  • Pipette
  • Funnel
  • Filter Paper
  • Flask
  • Water in Wash Bottle
  • Drying Oven
  • Electronic Scale

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Gloves must be worn at all times.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.

Procedure

  1. Obtain a sample of contaminated water (approximately 25 mL) and record its volume as precisely as possible. Pour the water into a clean beaker.
  2. Obtain a small amount of the solution you chose in pre-lab question #1 and slowly add it to the contaminated water sample until all of the Pb2+ has precipitated (see Pre-lab question #5). Be sure to record the concentration from the label.
  3. Weigh a dry piece of filter paper, then use it to filter out your solid product, wash it and heat it to constant mass. Record your final mass of solid product.
  4. Discard all waste products in the labeled containers in the fume hood.

Data

Volume of Contaminated Water Sample (mL)

Mass of Filter Paper (g)

Mass of Filter Paper and Product #1 (g)

Mass of Filter Paper and Product #2 (g)

Mass of Filter Paper and Product #3 (if needed) (g)


Calculations
Be sure to show all work, round final answers to the correct number of significant figures and include a unit on your final answer.

  1. Calculate the moles of solid product isolated (Hint: See pre-lab #4)
  1. Determine the moles of Pb2+ isolated.
  1. Calculate the mass of Pb2+ isolated.
  1. Calculate the molarity of Pb2+ in the original water sample.

Discussion Questions

  1. How does the Pb2+ concentration in this contaminated water sample compare to the EPA limits? The EPA limit is 15 ppm or 15 μg/L. Is this sample “dangerous”? Explain.
  1. Do some research to determine the lead drinking water standards in your state. How does your sample compare? Be sure to show any math needed to get your concentration values into the same units.
  1. The “limiting reactant” in a reaction is defined as the reactant that ran out first, thus “limiting” the amount of product formed. Which reactant do you think was “limiting” in this reaction? Explain.
  1. Explain how you were able to determine the mass of Pb2+ from the mass of solid product.
  1. Lead levels found in Flint, Michigan at the peak of the Water Crisis were generally around 20μg/L.
    1. Calculate the mass of lead (in grams) that you would find in 100.0 mL of Flint water.
    1. Would our method of lead testing be effective for measuring lead levels in Flint? Explain.