# Measurement Unit - WeissChem

Measurement Unit Accuracy vs Precision A Look at the Vocabulary The terms Accuracy and Precision are often used interchangeably

within the English language (but outside of the Science world). But, in the Science realm, we have very different definitions for the two terms. Complicating the discussion a bit is the

fact that Chemistry actually has two different uses for the term Accuracy. Accuracy 1 Definition st Defined as how close an experimental measurement is to an accepted value.

By accepted value, we mean the theoretical correct answer. for example Suppose that in the lab, you calculated the density of Unknown A as 2.68 grams/cm3 The correct value for the density of that metal is 2.70 grams/cm3 Since your calculated measurement is very close

to the actual value, we would say that your result was accurate. Accuracy Definition #2 This definition is used to describe the possible accuracy that one may measure with a given piece of

laboratory equipment. This description will typically report to what decimal place a given measurement can be reported.

Determining This Accuracy The rule of thumb that is used is that we will always imagine there to be 10 lines between the smallest lines on the device. The decimal place created by those imaginary lines establishes the possible accuracy of the measuring device. Now what about Precision? Precision refers to whether a SET OF

MEASUREMENTS are CLOSE IN VALUE to EACH OTHER. It is important to note that the measurements in question may or may not be accurate. All that you are doing here is comparing a set of measurements to each other. Consider the visuals on the following slides

Is the person throwing the dart in this drawing Accurate ? Why or why not? Is the person throwing the dart in this drawing Precise ? Why or why not?

Are the results represented here Accurate ? Why or why not? Are the results represented here Precise ? Why or why not?

So what about the accuracy and precision of these results? The Determination of Experimental Error The Two Quantities to Know: Experimental Value: This is the result that

is determined by YOU. You actually measured it. You had your hands on the equipment to determine its value. Theoretical Value This is the correct answer. It is not typically based on measurements. You can usually find it in a book of correct answers. Experimental Error calculates a difference

between YOUR EXPERIMENTAL VALUE and the THEORETICAL VALUE. Exp. Error = E - T T X 100 % Where E represents Experimental

and T represents Theoretical. an example A chemistry student performs a procedure to determine the density of an irregular solid object. After completing the process, the student calculated a value of 7.55 grams/cm 3. The object was pure iron and this metal has an established density of 7.86 grams/cm3. What was the students experimental error in this lab? Experimental

Theoretical Object to solve Exp. Error = E - T T =

X 100 % 7.55 - 7.86 7.86 = 3.94 % X 100 %

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