Chemistry Notes - Chemistry Chapter 2 measurements in...

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Chemistry Chapter 2 – measurements in chemistry Physical Quantities Two Parts Number Unit Both Parts are required for a measurement Quantity does not depend on unit Number does i.e. 1 mile = 5280 feet Same distance…will always have a number and a unit.  SI units * International system of Units Quantity/Unit - Mass/kg - Length/m - Temperature/Kelvin (K) - Time/second (s) – speed of light… Sometimes use other units * SI unit of volume is m cubed – m^3 – most common Huge volume (size of refrigerator - Use the liter 1 L = 0.001 m^3 - kg is large for lab-scale chemistry Usually use gram 1 g = 0.001 kg Use Prefixes to “scale” units - sometimes units are too big or too small - need tens, hundred, thousands, etc.. - Use Standard prefixes - common, milli – 10^-3, Micro 10^-6, Nano – 10^-9
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Converting units with prefixes - 1234.5 m = 1.2345 km Sig figs - Measurements have associated error - limit of measurement device - i.e. ruler marked to 1mm - Reporting more precision than measure is useless - Even misleading - Simplest way of handling this is significant figures - significant digits Determining number of significant digits 1. all nonzero dig its are significant 2. zeroes between nonzero digits are significant 3. zeroes to the left of the first nonzero digit are not significant 4. Zeroes to the right of the last nonzero digit - are significant if they are to the right of the decimal - may be significant if they are to the left of the decimal point EXAMPLES - 1203.43 – 6 sig figs - 0.000345000 – 6 sig figs o Scientific Notation - Numbers are often too big or too small to be convenient - 12503000 atoms or 0.0000000213 g  - Use place value to write more conveniently - Scientific Notation - 125, 030,000 = 1.2503 x 10^8 o Writing scientific notation - each number as a mantissa and an exponent - usually choose exponent to make mantissa between 1 and 10       1.2503 x 10^8       mantissa is 1.2503        exponent is 10^8 - Move Decimal point - make number between 1 and 10 - count number of places decimal point is moved
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Lecture 3 – 1/25/08 Adding Quantities If numbers are written in scientific notation, exponents must be the same Align Decimal point and add Last sig fig inn result is the “leftmost” sig fig in the addends. 2.34 x 10^3 +  1.2753 x 10^2  23.4 x 10^2 +1.2753 x 10^2  When rounding…you use the number with the least amount of sig figs and  then round to that i.e. 23.4 you round to 3 sig figs Multiplying Quantities Result will have units which are the product of the units of the multiplicands  Multiply numbers Result will have as many sig figs as the multiplicand with fewer sig figs …..  2.4 x 10^3 m x 3.42 g/m …. Units are in grams only 2 sig figs 8.2 x 10^3 g  
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