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The Molecular Weight of a Polymer

Grades: 11-12 AP Chemistry
Author: Antonello Cotugno
Source: This material is based upon work supported by the National Science Foundation under Grant No. EEC- 1542358.


Abstract

The lesson is designed to be used at the beginning of an AP or general chemistry college course.

The molecular weight of a polymer can be determined using several methods such as size exclusion chromatography and dilute solution viscometry.

Dilute solution viscometry is used to review a number of math concepts that are integral to an AP chemistry course, such as graphing, calculating the slope of a line, preparing and using standard curves, deriving and using constants and solving for an unknown in an equation. These concepts should be reviewed with students prior to assigning the lab activity.

In this lab activity, students prepare solutions of polyethylene glycol of known molecular weight at different concentrations. The viscosity of each solution is determined using a U-tube viscometer. Data is graphed to produce a standard curve from which the values of the Mark-Houwink constants (k’ and a) are determined.

Viscometry data is then measured for several solutions of polyethylene glycol of unknown molecular weight. Using the values for the constants determined from the standard curve, and the Mark-Houwink equation, the molecular weights of the unknown PEG solutions or the concentrations of the solutions can be determined.


Objectives

What should students know as a result of this lesson?

  • Polymers are made of repeating units called monomers. The number of repeating units determines the properties of the molecule such as molecular weight.
  • Intermolecular attractions include dipole-dipole interactions, hydrogen bonding and London dispersion forces.
  • That experimental design includes working with different types of variables, and the use of math concepts such as dimensional analysis, interpreting graphs, deriving and using constants, solving for a variable in an equation

What should the students be able to do as a result of this lesson?

  • Describe a polymer in terms of the type and number of monomer units and their intermolecular attractions.
  • Identify independent, dependent and controlled variables in an experiment
  • Construct and interpret graphs and use appropriate math concepts to solve problems
  • Design an experiment to determine the concentration of an unknown solution using a standard curve

Materials

  • MiraLaxR 3350 laxative– 10 % (mass/volume) aqueous solution (PEG standard)
  • 10 % (mass/volume) aqueous solution of two store brands of PEG 3350 laxative
  • Two discount store brands of PEG 3350 laxative
  • Glass capillary tube viscometer (1.2 mm)
  • Stopwatch
  • 100 mL beaker
  • 50 mL beakers (4)
  • 25 mL measuring cylinder
  • Ring stand and utility clamp
  • Suction bulb

Procedures

Engagement

Day 1:

  • Pre-test on polymers and chemistry math concepts
  • Engagement: 2-3 hands-on activities/demos with polymers.

Assessment: WebQuest about polymers and class discussion follows of the science of the fun activities

Exploration

Days 2-3:

  • PowerPoint lecture about Dilute Solution Viscometry
  • Molecular Weights of Polymers lab

Assessment: Students complete the pre-lab questions, data tables, show work for calculations, construct properly labeled graphs, and answer analysis and conclusion questions-including designing and implementing a procedure for determining the concentration of an unknown solution using viscometry data.

Explanation

Day 4:

  • Discussion - As a whole class, questions about the lab are posed and answers solicited from other class members with guidance from the instructor with the goal of completing the lab report

Assessment: Final Lab Report

Elaboration

Day 5:

  • Discussion of the importance of concepts from lab such as calculating slope of a line, percent error and statistics, significant figures and experimental design to topics in the AP chemistry course.
  • Presentation of prior AP questions relating to these topics.

Day 6:

  • Post-test

Prerequisites

Lab safety, graphing and slope, significant figures, conversions, experimental design.


Best Teaching Practices

  • Scientific literacy
  • Authentic problem-based or issue-based learning
  • Inquiry approach
  • Conceptual understanding of problem-solving approach
  • Real life situations and problem solving
  • Learning cycle

Alignment with Standards

National Science Education Standards:

  • NS.0-12.1 Science Inquiry
  • NS.0-12.1: Personal and Social Perspectives

AP Learning Objectives - Big Idea 2:

  • LO 2.29 The student can create a representation of a covalent solid that shows essential characteristics of the structure and interactions present in the substance.
  • LO 2.30 The student is able to explain a representation that connects properties of a covalent solid to its structural attributes and to the interactions present at the atomic level.
  • LO 2.31 The student can create a representation of a molecular solid that shows essential characteristics of the structure and interactions present in the substance.
  • LO 2.32 The student is able to explain a representation that connects properties of a molecular solid to its structural attributes and to the interactions present at the atomic level.

NGSS Standards:

  • HS-ETS1-1: Analyze a major global challenge to specify qualitative and quantitative criteria and constraints for solutions that account for societal needs and wants.
  • HS-ETS1-2: Design a solution to a complex real-world problem by breaking it down into smaller, more manageable problems that can be solved through engineering.
  • 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.
  • HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.

Ohio Standards:

Science Inquiry and Application

During the years of grades 9 through 12 all students must use the following scientific processes with appropriate laboratory safety techniques to construct their knowledge and understanding in all science content areas:

  • Identify questions and concepts that guide scientific investigations;
  • Design and conduct scientific investigations
  • Use technology and mathematics to improve investigations and communications;
  • Formulate and revise explanations and models using logic and evidence (critical thinking);
  • Recognize and analyze explanations and models; and
  • Communicate and support a scientific argument.

Content Knowledge

It is assumed that the average high school student knows nothing about polymers. Students should already be familiar with the following topics:

  • Dimensional analysis, significant figures, scientific notation, and statistics
  • Intermolecular forces-dipole-dipole, hydrogen bonding, London dispersion forces
  • Basic lab techniques
  • Constructing and interpreting graphs, calculating slope of a line.
  • Solving for an unknown in an equation
  • Experimental design, including working with variables (independent, dependent, controlled)

This lesson provides students with an opportunity to apply these math and science concepts in preparation for the requirements of the AP chemistry curriculum


Safety

  • Materials used in this lab activity (poly ethylene glycol, distilled water) are non-hazardous and water soluble.
  • Students should employ safety techniques for working with glassware.

Applications

  • We often need to evaluate if different brands of a product meet our needs (cost, quality, effectiveness etc.). For example: does the cheaper generic brand of a drug work as well as the name brand.
  • Analytical chemistry is an important quality control technique used in medicine, and the food and materials industries.
  • Math principles can be important in real-world situations such as interpreting and predicting stock performance given in the form of a graph or determining how much topsoil is needed to cover an area in a garden by solving for a variable in an equation

Assessment

Students complete the lab report and take a test over the material in this lesson


Other Considerations

Grouping Suggestions:

  • Students work in pairs

Pacing/Suggested Time:

  • This lesson will take several days to complete

Printable PDF Worksheets


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