Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107

Fundamentals of Chemistry

Fall 2009

Lecture Notes: 27 August

© R. Paselk 2005
 
     
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What is Chemistry?

Why Chemistry is often considered the "central science." Examples.

Chemistry is the study of matter and its transformations.

More specifically, chemistry is the scientific study of matter. So what do we mean by science? Two common "definitions":

Matter

What is matter? Stuff. Has mass and occupies space.

Mass: The measure of quantity for matter. Mass is the property of matter resulting in its inertia and and attraction via gravity.

Matter has both physical properties and chemical properties. These are properties which do not depend on the quantity of substance and therefore they can be used to identify a substance (sometimes referred to as intensive properties).

States of Matter. Matter can exist in three states under earth-surface conditions:

A fourth state of matter commonly occurs under special conditions: a plasma. A plasma is an ionized fluid - can be contained by magnetic fields.

Measurements

Accuracy and Precision

First we need to define and distinguish between two terms: accuracy and precision. Consider the two targets below:

Images of two targets showing a closely grouped hits vs. a scattered set of hits.

Which target is the work of the better marksman? I would say B, because she always hits nearly the same place - all we have to do to get all of her shots in the center is to adjust her sights. On the other hand, A is scattered all over. Sure he hit the center once, but, on average he needs lots of shots to do it, adjsuting his sights will do us no good!

For the most part, it is more important to be precise, than it is to be accurate, since we can always adjust our instrument, or our data, to bring the results to the proper value.

Exponential or scientific notation

It is often convenient to express numbers in exponential or scientific notation to indicate significant figures, and to just avoid writing the huge numbers of zeros we often run into in the natural world. [examples] See lab book exercises.

Significant Figures

For measurements we want to be sure we convey the precision (repeatability) of our measurements using significant figures. [examples] See lab book exercises.

Significant Figures and Calculations: Two basic sets of rules:

  1. Addition/Subtraction rule: Significant figures are determined by looking at the decimal place of the numbers being added or subtracted. The number with the "least decimal places" determines the decimal place of the answer, e.g. if we add 1,216,956 to 214.879, the first number has the fewest decimal places, so the answer is rounded off to the 1's place: 1217170.879 goes to 1217171.
  2. Multiplication/Division rule: In this case we count the digits. The number with the fewest significant digits determines the number of significant digits in the answer.

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Last modified 28 August 2008