Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2015

Lecture Notes 4: 28 January

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SI Units (metric system)

SI Units: The metric system originated around the French Revolution as a rational system of measurements to rescue France from the chaos of pre-revolutionary measurements and thus prevent tax collectors from cheating.

Wanted to base system on "natural" universal standards. Thus for length they chose the size of the Earth: specifically the meter was defined as one ten-millionth (10-7) of the Earth's meridian (line from the S to the N pole) through Paris. For mass the Kilogram was defined as the mass of a cube of water 0.1 meter on a side. Of course these are not convenient, so standards were quickly created: the meter became the distance between two lines on a platinum-iridium bar stored in a vault in Paris, while the kilogram became a cylindrical mass of platinum-iridium stored in the same vault.

Today the various units are defined by international agreement to give the SI (Systéme International) units:

SI Prefixes

Note Table 1.2 in your text (p 10). You should know (memorize) and be able to interconvert the prefixes in the table below:

Prefix Symbol Magnitude
tera-
T
1012
giga-
G
109
mega-

M

 106
kilo- k 103
base   100
deci- d 10-1
centi- c 10-2
milli- m 10-3
micro-

mu (or mc)

10-6
nano- n 10-9
pico- p 10-12
fempto- f 10-15

Memorize: 1 mL = 1 cm3; 1 inch = 2.54 cm (defined); 1 liter is about 1 quart; density of water = 1 g/mL; 0° C = 32 °F, 100°C = 212 °F, -40 °C = -40 °F.

Temperature

Look in your text for conversions between °C and °F and example problems.

Matter

Recall that chemistry is the study of transformations of matter. But what is matter? Matter is stuff. It has mass and occupies space.

Mass

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

Do not confuse mass and weight. Weight is the force acting on an object due to gravity. We often interchange these terms in conversation, but they are quite different - you have the same mass whether you are weightless in space on here on Earth (taking a shuttle flight is no substitute for a diet!). To confuse us further we call the determination of mass "weighing"!

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.

Density

Density is defined as the mass of a given volume of a substance: Density = mass/volume. Note that this weeks laboratory exercise give practice in Density, significant figures etc.

Let's try some density problems. First note that the units of density are g/cm3 or g.cm-3.

Known: Density = mass/volume, generally expressed as g/mL = g/cm3

Solve: (35.987 g) / (20.0 mL) = 1.79935 g/mL

note that the units are those of density so we are confident we set it up correctly.

How about sig figs? Use multiplication/division rules, so count: 3 for 20.0 and 5 for 35.987, therefore should have three sig figs:

1.79935 g/mL = 1.80 g/mL

Extra Example: Using a jewelers balance a student found that a coin weighed 2.34 carats in air. By weighing it again submerged in water she found it had a volume of 0.034 mL. What is its density? (1 carat = 200 mg, defined)*

Known: 1 carat = 200 mg (defined), density is g/mL

Solve: (2.34 carats)(200 mg/carat)(1 g/1,000 mg) / 0.034 mL = 13.764706 g/mL

How about sig figs? Both conversion factors are defined, so exact. Two measurements: 2.34 and 0.034 = 3.4 x 10-2. Thus the answer will have only two sig figs since using counting rule - least number of sig figs.

13.764706 g/mL = 14 g/mL

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© R A Paselk

Last modified 28 January 2015