Ch. 3 Scientific Measurement Ch. 3 Scientific Measurement

Measurement  Quantitative information  Need a number and a unit (most of time)  Represents a quantity  For example: 2 meters 2 is number 2 is number Meters is unit Meters is unit Length is quantity Length is quantity  Units compare what is being measured to a defined measurement standard

SI Measurement  Le Systeme International d’Unites : SI  System of measurement agreed on all over the world in 1960  Contains 7 base units  We still use some non-SI units

Important SI Base Units QuantitySymbolUnitAbbreviation Lengthlmeterm Massmkilogramkg Timetseconds TemperatureTKelvinK Amountnmolemol

Prefixes  Prefixes are added to the base unit names to represent quantities smaller or larger Mmega10 6 1,000,000larger kkilo10 3 1,000larger ccenti /100smaller mmilli /1000smaller μmicro /1,000,000smaller

Length  SI unit: m  use cm a lot too cm  km is used instead of miles for highway distances and car speeds in most countries km

Mass  Measure of the quantity of matter  SI unit: kg  use g a lot too g  mass vs. weight weight is the measure of gravitational pull on matter weight is the measure of gravitational pull on matter mass does not depend on gravity mass does not depend on gravity on a new planet, mass would be same but weight could change on a new planet, mass would be same but weight could change

Temperature Conversions Fahrenheit to Celsius

Celsius to Kelvin conversions → CK ←

Temperature Conversions

Example  What is 32°F in Kelvin?  freezing point of water!

Example  What is 298 K in Fahrenheit?

Derived SI Units  come from combining base units  combine using multiplication or division Example:Area: A = length x width = m x m = m x m = m 2 = m 2

Volume  amount of space occupied by object  SI: m 3 = m x m x m  use cm 3 in lab a lot cm 3cm 3  non-SI: 1 liter = 1dm 3 = 1000cm 3 1 liter = 1000 mL 1cm 3 = 1mL

Density  ratio of mass to volume  SI:  Other units: g/ cm 3 or g/ mL  characteristic property of substance (doesn’t change with amount ) because as volume increases, mass also increases  density usually decreases as T increases exception: ice is less dense than liquid water so it floats

Example A sample of aluminum metal has a mass of 8.4 g. The volume is 3.1 cm 3. Find the density. KnownUnknown m = 8.4 gD = ? V = 3.1 cm 3

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 Dimensional Analysis

Conversion Factors  can be multiplied by other numbers without changing the value of the number  since you are just multiplying by 1

Example 1 Convert 5.2 cm to mm cm to mmcm to mm 5.2 cm= 5.2 x 10 1 mm = 52 mm cm= 5.2 x 10 1 mmcm= 5.2 x 10 1 mm  Known:100 cm = 1 m 1000 mm = 1 m  Must use m as an intermediate

Example 2 Convert kg to mg kg to mgkg to mg kg = x 10 6 mg= 20,000 mg kg = x 10 6 mgkg = x 10 6 mg  Known:1 kg = 1000 g 1000 mg = 1 g  Must use g as an intermediate

Example 3 Convert 500,000 μg to kg μg to kgμg to kg 500,000 μg = 500,000 x kg= kg μg = 500,000 x kgμg = 500,000 x kg  Known:1,000,000 μg = 1 g 1 kg = 1000 g  Must use g as an intermediate

Advanced Conversions  One difficult type of conversion deals with squared or cubed units Example  Convert 3 dm 3 to cm 3 1dm =10 cm 3 dm 3 = 3 x 10 cm x 10 cm x 10 cm= 3000 cm 3

Example  Convert: 2000 cm 3 to m 3 OR Known: 100 cm = 1 m cm 3 = cm x cm x cm m 3 = 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

Example Convert 350 g/ mL to kg/L  Top first 350 g to kg 350 g= 350 x kg=.35 kg  Bottom part after 1mL to L 1mL= 1x L= L  Result: 350 g/ mL =.35 kg/ 0.001L= 350kg/ L

Combination Example Convert 7634 mg/m 3 to Mg/L  Top 7634 mg= 7634 x Mg= Mg  Bottom 1m= 10 dm 1 m 3 = 10 dm x 10 dm x10 dm= 1000 dm 3 = 1000 L  Result: 7634 mg/m 3 = Mg/ 1000 L= Mg/L = x Mg/L

Accuracy vs. Precision  Accuracy- closeness of measurement to correct or accepted value  Precision- closeness of a set of measurements

Accuracy vs. Precision

Percent Error vs. Percent Difference  Percent Error: Measures the accuracy of an experiment Measures the accuracy of an experiment Can have + or – value Can have + or – value

Percent Error vs. Percent Difference  Percent Difference: Used when one isn’t “right” Used when one isn’t “right” Compare two values Compare two values Measures precision Measures precision

Example  Measured density from lab experiment is 1.40 g/mL. The correct density is 1.36 g/mL.  Find the percent error.

Example  Two students measured the density of a substance. Sally got 1.40 g/mL and Bob got 1.36 g/mL.  Find the percent difference.