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:
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-
106 kilo- k 103 base 100 deci- d 10-1 centi- c 10-2 milli- m 10-3 micro-
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.
Look in your text for conversions between °C and °F and example problems.
Recall that chemistry is the study of transformations of matter. But what is matter? Matter is stuff. It has mass and occupies space.
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 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)*
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© R A Paselk
Last modified 28 January 2015