Chem 109 - General Chemistry - Spring 2015
Lecture Notes 4: 28 January
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:
- Length: the meter (m) is defined as the distance light travels in a vacuum in 1/299,792,458 sec (note that this is truly universal: in principle it can be determined by anyone, anytime, anywhere in the Universe).
- Mass: the kilogram (kg) however is still based on the International Prototype Kilogram in Paris and the derived standard kilogram standards held by governments around the world.
- Time: the second (s) is defined today as the duration of 9,192,631,770 periods of the radiation of two hyperfine levels of the ground state of the cesium 133 atom.
- Amount of substance: the mole (mol) is defined as the number of atoms in 0.012 kg (defined, so sig figs not restricted) of carbon 12 atoms.
- Temperature: the kelvin (K) is defined as 1/273.15 of the thermodynamic temperature of the triple point of water.
- Electric current: the ampere is defined as the the current which carries one coulomb (6.24146 x 1018 times the charge on an electron or proton) of charge through a conductor in one second.
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- |
 (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).
- Physical properties of substances can be observed without, in principle, changing their compositions. Physical properties include mass, color, density etc. Note that physical changes such as melting, cutting, etc. do not change composition. When you perform a physical operation such as melting or cutting, you have changed the object, but you still have the same kind of stuff - chop a wooden stick in half, you have two sticks, not a new kind of thing like a rock or piece of metal. Physical operations are also often readily reversible - if you melt ice the water can be readily returned to ice, though not necessarily the same shape, it is obviously the same stuff!
States of Matter. Matter can exist in three states under earth-surface conditions:
- Solid: definite shape and volume (Crystals vs. super-cooled liquids or glasses)
- Liquid: definite volume, but no defined shape - will fit to container etc.
- Gas: no definite shape or volume - will fill whatever container they are in.
- both liquids and gases are fluids.
A fourth state of matter commonly occurs under special conditions: a plasma. A plasma is an ionized fluid - can be contained by magnetic fields.
- Chemical properties of substances describe behaviors which lead to changes in composition. Chemical properties describe reactivity under various circumstances (does it burn in air, react with acids or bases, corrode in sea water etc.) Note that chemical changes result in different compositions. If you burn a stick you no longer have wood - you have created a new set of kinds of stuff such as carbon dioxide and water, a gas and a liquid completely unrecognizable as even related to wood.
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.
- A student found that 20.0 mL of a liquid weighed 35.987 g. What is its density?
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
