Fall 07 CHEM 136 MId-term Exam Review

Fall 07 CHEM 136 MId-term Exam Review - Review Approaches...

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Review Approaches for CHEM 136 Exams–Mid-term and Final Each of the first five experiments to be covered on the mid-term are described below. A supplement will be provided after Thanksgiving addressing the remaining five experiments to be covered on the final exam. Descriptions address the following: The major conclusion of the experiment When relevant, the major concept explored Facts used, or developed, which are to be retained The skills or techniques developed in the experiment The data or observations leading to the conclusion When necessary, how required calculations were to be performed How the concept, or conclusion was applied in post-lab questions or subsequent experiments A good way to get the most out of making use or it, or review sessions with the TA or questions asked at office hours, would be to review your graded lab reports and see how ideas presented here jive with the answers you gave in those reports (even if you received a good grade on the report); questions related to differences between these responses may be the most informative things you could ask. Expt 1: Accuracy and Precision in Volumetric Measurements Conclusion: The relative accuracy and precision of different types of volumetric glassware is a function of the number of significant figures to which volume can be known (read plus estimated), given the markings on the glassware. Facts: The “maximum number of significant figures” which can be read from any measuring tool includes all figures read directly from the markings on the tool plus one additional estimated figure. The mass of each sample of water measured by each type of glassware was the only piece of data which was “known” in each measurement. Readings from the glassware were subject to experimenter error. “True” volume measured could be calculated by applying the density of water at the experimental temperature to the mass of the sample measured. Skills: Proper use of the analytical balance Experience reading volume from markings on Mohr pipettes and burettes Use of pipette pumps and bulbs How data led to conclusions: The mass of two separate 20 mL samples of water were measured out and weighed with an analytical balance. That mass was converted to actual volume by dividing by the density at the experimental temperature. Comparison of this calculated “true” volume with the volume thought to have been read from the volume markings allowed the “accuracy” of the glassware to be calculated. Comparison of the two “true” volumes measured out by each piece of glassware allowed determination of the “precision” of the glassware.
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Note: A common error made by students was to compare the first 20 mL sample measured in the beaker or graduate cylinder with the total of 40 mL present when the second 20 mL sample had been added.
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Fall 07 CHEM 136 MId-term Exam Review - Review Approaches...

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