Purpose: To determine uncertainty in measurements for various chemistry laboratory apparatus and to consider uncertainty when constructing and analyzing a graph.

Performance Goals

 Determine the appropriate number of decimal places to use in recording measurements

 Utilize calibration scales to determine the accuracy and precision of a laboratory instrument or tool

 Apply statistical functions in the analysis of experimental data

 Construct a graph by hand and by the use of Excel based on data collected

 Analyze a graph to determine the accuracy and precision of the data collected

Introduction

Laboratory work involves making reliable measurements. Reliability of a measurement depends primarily upon two factors: accuracy and precision. These two factors depend upon the quality of the measuring device, the procedure, and the technique of the operator. 

The accuracy of a measurement is how close the measurement is to the correct or commonly accepted value. When we repeat the same measurement several times, the values obtained are not usually exactly the same. The precision is how close those measurements are to each other, that is, how reproducible the measurements are.

This is illustrated by the figure below showing the hits made by darts on a target. Good accuracy would be having a dart hitting bull’s eye. Good precision would be hitting the same spot consistently, but not necessarily hitting bull’s eye.

In this experiment we will begin by examining the graduations marked on various laboratory devices. The finer the graduation, the more precise is the measurement (that is, more reliable digits can be recorded). The general rule is to record to one-tenth of the smallest increment in the scale of the measuring device by estimating the last digit. Thus, the last digit of any measurement has a degree of uncertainty. Consider the two rulers (I and II) in Figures 1 and 2 below. In the case of Ruler I the smallest increment is 0.1 cm and all readings should be recorded to one-tenth of 0.1 cm, which is 0.01 cm (that is, to 2 decimal places). For Ruler II, however, the smallest increment is 1 cm, and all readings should be recorded to onetenth of 1 cm, which is 0.1 cm (that is, to 1 decimal place). You should follow this rule throughout the semester whenever you are making a measurement, unless the procedure specifically tells you otherwise.

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