II. Units of Measurement (p. 33 - 39) CH. 2 - MEASUREMENT II. Units of Measurement (p. 33 - 39) C. Johannesson

A. Number vs. Quantity Quantity - number + unit UNITS MATTER!! Suppose a chef writes a recipe listing ingredients such as 1 salt, 3 sugar, and 2 flour. You wouldn’t be able to follow the recipe without further information, such as whether the 3 represented teaspoons, tablespoons, cups, ounces, etc...the same is true of measurements….they represent quantities and therefore require a number and a unit. UNITS MATTER!! C. Johannesson

In chemistry, we focus on five… A. Number vs. Quantity Scientists all over the world have agreed on a single measurement system called the SI System, or Le Systeme International d’Unites. SI now has seven base units, and most other units are derived from these seven. In chemistry, we focus on five… People used to measure off distances by marking the number of food lengths it took to cover that distance. This didn’t work out very well because the number of foot lengths varied with each individual’s foot. Once a standard for foot length was agreed upon, confusion as to the real length was eliminated. It no longer mattered who made the measurement as long as the standard was correctly applied. C. Johannesson

B. SI Units Quantity Symbol Base Unit Abbrev. Length l meter m Mass m kilogram kg Time t second s Temp T kelvin K Amount n mole mol C. Johannesson

B. SI Units Prefixes added to the names of SI base units are used to represent quantities that are larger or smaller than the base units. C. Johannesson

B. SI Units Prefix Symbol Factor kilo- k 103 hecto- h 102 deka- da 101 BASE UNIT --- 100 deci- d 10-1 On board… -Write Meaning -Example’s centi- c 10-2 milli- m 10-3 micro-  10-6 nano- n 10-9 C. Johannesson

B. SI Units For Example: -The SI unit for length is the meter. -1 m = width of a doorway. -1 km = 1000 meters; used to express highway distances. -1 cm = width of a paper clip. *Brainstorm more Comparisons of Units* C. Johannesson

Mass vs. Weight Mass is the amount of matter present in a body and remains the same wherever the body might be. Weight is a measure of the gravitational pull on matter and differs by place. For example, objects weigh less on the moon where gravity is lower. C. Johannesson

C. Derived Units Derived Units are made up of a combination of SI base units. Quantity Quantity Symbol Unit Unit Abbreviation Derivation Area A Square meter l x w Volume V Cubic meter L x w x h Density D Kilograms per cubic meter C. Johannesson

Question… What is the volume, in cubic meters, of a rectangular solid that is 0.25 m long, 6.1 m wide, and 4.9 m high? 7.5 m3 C. Johannesson

C. Derived Units C. Johannesson   -SI unit for density is derived from the base units for mass and volume. -Doesn’t depend on size of sample because as mass increases, volume increases proportionately; therefore, the ratio of mass to volume is constant. -Does depend on temperature because when you increase the temperature of a substance, it expands, so the volume increases. When you divide by a larger number, you are going to get a smaller answer, so that's why density decreases. Buildings were designed to allow for expansion and contraction, as were sidewalks which is why they have cracks in between them. C. Johannesson

C. Derived Units Densities of Some Familiar Materials Solids Density at 20⁰C (g/cm3) Liquids cork 0.24 gasoline 0.67 ice 0.92 Ethyl alcohol 0.791 sucrose 1.59 water 0.998 lead 11.35 Sea water 1.025 -Densities in table explain why an object made of cork feels lighter than a lead object of the same size. -The numerical relationship dictates what floats. Ask students whether ice would sink or float when placed in gasoline? -Real-world application: carbon dioxide gas can be used to put out a fire because it is more dense than oxygen and thus sinks, blocking out oxygen from the fire. C. Johannesson

D. Density V = 825 cm3 M = DV D = 13.6 g/cm3 M = (13.6 g/cm3)(825cm3) An object has a volume of 825 cm3 and a density of 13.6 g/cm3. Find its mass. GIVEN: V = 825 cm3 D = 13.6 g/cm3 M = ? WORK: M = DV M = (13.6 g/cm3)(825cm3) M = 11,220 g C. Johannesson

D. Density D = 0.87 g/mL V = M V = ? M = 25 g V = 25 g 0.87 g/mL A liquid has a density of 0.87 g/mL. What volume is occupied by 25 g of the liquid? GIVEN: D = 0.87 g/mL V = ? M = 25 g WORK: V = M D V = 25 g 0.87 g/mL V = 29 mL C. Johannesson