LDPE (low density polyethylene, #4) is a petroleum-based polymer used to make flexible bottles, film — Experimental Design / Density and Thickness Chemistry Question
Problem Context
LDPE (low density polyethylene, #4) is a petroleum-based polymer used to make flexible bottles, films, and plastic containers. Given water, ethanol (density = 0.789 g·mL–1), and the equipment provided, devise and carry out an experiment to precisely determine the thickness of the LDPE samples provided
1. Give a brief description of your experimental plan.
2. Record your data and other observations.
3. Calculations and Conclusions.
Your reported thickness of LDPE =
Model Answer
Excellent Students Results:
Student proposed a clear, detailed procedure for determining the thickness of the LDPE sheet, recognizing that measuring the volume of such a sheet directly would not be possible because of the small volume. Excellent procedures invariably involved measuring the density of the plastic; good methods included making a series of ethanol-water mixtures and interpolating the mixture of neutral buoyancy, or starting with one liquid and adding the other until neutral buoyancy was achieved. Density of the neutrally buoyant liquid was measured either by using the weighted average of ethanol or water, or by direct measurement of the mass of a known volume of the liquid.
Student performed several buoyancy trials, either using a variety of water-ethanol volume ratios in a series of standards, or by redetermining the point of neutral buoyancy. Results were clearly displayed in a data table. Area and mass of LDPE piece(s) were measured in duplicate. Calculations are clearly shown using proper unit measurements and significant figures in final answers. Student demonstrated knowledge of the assumptions used in calculation (for example, the assumption of additive volumes if density of the neutrally buoyant mixture was calculated rather than measured directly). Final value for thickness was within 20% of the accepted value.
Average Student Results:
Measurement of density was proposed, but not clearly thought out; or, less precise procedures for determining volume directly (e.g., by displacement of liquid in the graduated cylinder) were proposed. Student made only qualitative (floats in water, sinks in ethanol) or grossly erroneous measurements of density. Only one trial was performed. Final value for thickness was within 40% of the accepted value.
Below Average Student Results:
Procedure was vague or unintelligible. Calculations were unclear or in error. Final value for thickness was over 40% off from the accepted value.