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CHE 101LEC General Chemistry Lecture University at Buffalo

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University at Buffalo

CHE 101LEC General Chemistry Lecture University at Buffalo

Units of Measurement
• SI Units:
o Uses a base unit for each quantity
o Base units for all other units
-We consider the base units for:
• Length in meter (m)
• Mass in kilogram (kg) *The amt of material in an object
• Temperature in kelvin (K) * measure of hotness or coldness in an object
Prefixes: Used to indicate the fractions or multiples of various units
• Convert the base units into units appropriate for item being measured
Ex: Milli- a 10-3 fraction = one thousandth of a unit
Ex:1 millimeter (mm) is 10-3 meter m)
1mm=1×10-3 m
1kg=1000g
*If something is exact, there is uncertainty associated with it
*Every prefix is related back to its base unit
Derived SI Units:
• Volume- most commonly used units
• M3 or cm3 (used for solids)
o Cube: L x W x H
• Liter (L), Milliliter (mL) (used for liquids)
o 1mL=1cm3 (Direct relationship)
• Density: amount of mass in a unit volume of a substance
o g/cm3 or g/mL
o Temperature Dependent (Density=mass/volume; d=m/v)
o Density of Water: 1.00 g/cm3 when iced, volume expands so density
becomes smaller. Therefore, ice floats in water, since less dense than
water)
Dimensional Analysis
• Dimensional analysis: procedure of changing units to ensure to get the
proper unit using conversion factors
o Conversion Factor: A fraction with numerator and denominator the
same quantity but different units
§ Given unit x (Desired unit/Given unit) = Desired unit
§ *Given units cancel out)
• Converting from one unit to another for the same measure
o 1 inch=2.54 cm
§ Two conversion factors: 1in/2.54cm or 2.54cm/1in
§ Example: 20.2 in to cm?
• (20.2 in) (2.54cm/1in)=51.3cm *Need inch in
denominator to cancel out with the 20.2 INCH
• Or (20.3in) (1in/2.54cm)=7.95in2/cm (Wrong method)
**We never start with the conversion factor! Always start with the number that has
one unit
Example: How many centimeters are there in 6.51 miles? Mi->cm
• 1mi=5280ft 1ft=12in 1in=2.54cm
• *Set up in a way which units cancel out
6.51mi (5280ft/1mi) (12in/1ft) (2.54cm/1in) =1047682944cm
3 SFs exact exact exact =1050000cm
=1.05 x 106 cm
=3 SFs
• Anything that comes from measurement is not exact, so need to count sig
figs)
Example: A bucket contains 4.65 L water. How many gallons of water is that?
• 1L=1.057qt 1gal=4qt
4.65L (1.057qt/1L) (1gal/4qt) =1.23 gal
3SFs 4SFs exact 3 SFs
*GO with the fewer sig figs (whichever relationship is not exact; memorize
chart; use sig figs)
Example: Convert 1.36 x 109 km3 to liters Km3 -> L
• 1L=10-3 m3 1km=103m4
• 1.36 x 109 km3 (103m/1km)(1L/10-3m3) =1.36 x 1021 L
• Raise conversion factor to same factor: (103m/1km)3 = 109m3
• Km3
Example: The volume of well is 40.0 ft3. How many kilograms of concrete will it take
to fill the well if the density of concrete is 2.85 g/cm3?
*Never start our equation with the conversion factor!
*1kg=1000g Ft3 -> kg (*Due to uncertainty from measurement, determine sig figs)
40.0 ft3 (12in/1ft)3 (2.54cm/1in)3 (2.85g/1cm3) (1kg/1000g) = 3228.12kg
*We use 3 sig figs so final answer is: 3230 kg or 3.23 x 103 kg
Relationship between a prefix and a base unit: 1kg=1000g -> example of an exact
relationship
Chapter 2: Atoms, Molecules, and Ions
Structures of Atoms

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CHE 101LEC General Chemistry Lecture University at Buffalo

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