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2.C Units of Measurement Making Observations 2 types of info: Quantitative-numerical (usually numbers) Qualitative- descriptive (usually words) Measurement Quantitative information (quantity) Need a number and a unit (most of time) For example: 7.5 centimeters 7.5 is number Centimeters is unit Length is quantity Units compare what is being measured to a defined measurement standard SI Measurement Systeme International d’Unites : SI System of measurement agreed on all over the world in 1960 Units are defined in terms of standards of measurement Le Important SI Base Units Quantity Length Symbol Unit l meter Abbreviation m Mass m kilogram kg Time t second s Temperature T Kelvin K Amount n mole mol Mass Measure of the quantity of matter SI unit: kg use g a lot too mass vs. weight weight is the measure of gravitational pull on matter mass does not depend on gravity on a new planet, mass would be same but weight could change Length SI unit: m use cm & km a lot too Derived SI Units come from combining base units Example: Area A = length x width A = 8cm x 7cm A = 56 cm2 Volume amount of space occupied by object SI: m3 = m x m x m use cm3 in lab a lot **Need to know: 1 cm3 = 1mL 1000mL = 1L Density ratio of mass to volume kg SI: mass Density volume m3 Intensive property of matter (doesn’t change with amount ) Example A sample of Al metal has a mass of 8.4 g. The volume is 3.1 cm3. Find the density. Convert 3 years to weeks then to days then to hours then to minutes then to seconds. Prefixes Prefixes are added to the base unit names to represent quantities smaller or larger M mega 106 1,000,000 larger k kilo 103 1,000 larger c centi 10-2 1/100 smaller m milli 10-3 1/1000 smaller μ micro 10-6 1/1,000,000 smaller Conversion Factors ratio that comes from a statement of equality between 2 different units every conversion factor is equal to 1 Example: statement of equality conversion factor 4quarters 1dollar Guidelines for Conversions consider what unit you are starting and ending with (know, want to know) always cancel units as you go Example 1 Convert 5.2 cm to mm Known: 100 cm = 1 m 1000 mm = 1 m Example 2 Convert 0.020 kg to mg Known: 1 kg = 1000 g 1000 mg = 1 g Example 3 Convert 500,000 μg to kg Known: 1,000,000 μg = 1 g 1 kg = 1000 g Advanced Conversions One difficult type of conversion deals with squared or cubed units Be sure to square or cube the conversion factor you are using to cancel all the units Example Convert: 2000 cm3 to m3 Known: 100 cm = 1 m cm3 = cm x cm x cm m3 = m x m x m Advanced Conversions Another difficult type of conversion deals units that are fractions themselves Be sure convert one unit at a time; don’t try to do both at once Work on the unit on top first; then work on the unit on the bottom Setup your work the exact same way Example Convert: 350 g/mL to kg/L No intermediate needed OR Known: 1000 g = 1 kg 1000 mL = 1 L Combination Example Convert: 7634 mg/m3 to Mg/L Known: 1000 mg = 1 g 1,000,000 g = 1 Mg 100 cm = 1 m 1 cm3 = 1 mL 1000 mL = 1 L