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Chemistry: SI Units & Conversions

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SI Base Units of Measure commonly used in chemistry Time: second (s) Length,distance: meter (m) Mass: kilogram (kg) –In class, we consider the “base unit” to be grams. Volume: liter (l) Temperature: Kelvins (K) note: no degree symbol –In chemistry lab, we use the Celsius temperature scale. ( °C)

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Derived Units A derived unit is one in which the units are combined mathematically. –Volume: cubic centimeter (cm 3 ) (cm x cm x cm) –Area: square meter (m 2 ) ( m x m) –Density: mass volume

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Converting Celsius to Kelvin Kelvins = C + 273°C = Kelvins - 273

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Prefixes for SI Units Prefixes for units smaller than 1: –m–micro ( ) 1 X or –m–milli (m) 1 X or –c–centi (c) 1 X or –d–deci (d) 1 X or Prefixes for units larger than 1: –k–kilo (k) 1 X 10 3 or 1,000

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Conversion Using the Factor-Labeling Method : (Dimensional Analysis) Problem: Convert cm to m Step 1: write down the starting measurement: number and unit! Step 2: Convert from this prefix to the base unit. (This means meters, liters or grams.) X Step 3: Cancel the units that divide each other out. Step 5 If you have arrived at the target unit, you are done! Be sure to include the unit and significant figures in your answer! = Step 4: Perform the math… divide by the numbers on the bottom, multiply by the numbers on the top! Starting unit Target unit

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Practice! Convert km to cm X ? X= The correct answer is 450 cm, but that’s only 2 sig figs…. ????? Use scientific notation to correct the sig figs! 4.50 x 10 2 cm

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