Basic Foundations
SI Units
SI Units
• The table below lists the base SI units for
different measurements:
SI Unit Prefixes
• Prefixes are used to allow convenient measurement over
vastly different scales
Mass vs. Weight
• Mass measures the amount of matter an object contains
• Weight measures a force; e.g., the gravitational pull on an object,
which depends on its mass and the medium surrounding it
• Mass remains constant no matter where you are, while weight
fluctuates (such as in space, on another planet, under water, etc…)
– For this reason, scientists use mass
• SI-unit for mass is the kilogram (kg), but the gram (g) is also often
used
• Chemists often use atomic mass units (amu) to deal with matter at
the atomic level
• Atomic mass units (amu) is used to express the
masses of atoms
1 amu = 1.6605378 x 10-24 grams
Note: 1/1.6605378x10-24 = 6.02214 x 10-23=NA
where NA is called Avogadro’s number
Temperature
Celsius (°C)
• Freezing point of
water at 1 atm:
0°C
• Boiling point of
water at 1 atm:
100°C
Kelvin (K)
• “Absolute scale”
• Lowest possible
temperature: 0K
K = °C + 273.15
• Fahrenheit is the usual temperature scale in America.
• The freezing point of water at 1 atm is 32°F
• The boiling point of water at 1 atm is 212°F
°F= (9/5 × temp in °C) + 32 °F
SI-Derived Units
• Many times it is necessary to combine SI
units to make other useful units
Volume may be derived by
length:
V = (length)3
Example: cm3
Or one may use liters (L)
1 dm3 = 1 L
1 cm3 = 1 mL
These relationships must be memorized
SI-Derived Units
• Derived units may also combine with
other SI or SI-derived units
• Density is a common measurement which
gives the ratio of mass to volume of an object
– The SI-derived unit is given as kg/m3
• Density may be seen in units:
– g/cm3 (solids)
– g/mL (liquids)
– g/L (gases)
m
d=
V
SI-Derived Units
• SI-Derived units come up frequently in
science, and may be more convenient
– Example: Force = mass x acceleration
• 1 Newton (N) = 1 kg· m· s-2
– Example: Energy
• 1 Joule (J) = 1 kg· m2 · s-2
• These derived units make expressing
measurements easier
Non-SI Units
• Non-SI Units may also be convenient
• The SI unit of pressure is the Pascal
– 1 N/m2
• Other units are common:
– Atm, bar, psi, Torr, mm Hg
F
P= 2
l
1 atm = 101325 Pa = 1.01325 bar
1 atm = 14.7 psi (pounds per square inch)
1 atm = 760 Torr = 760 mm Hg
Chemistry Coffee Break
Finding Absolute Zero
• SI unit of temperature is the Kelvin
• The Kelvin scale is named after William Thomson, 1st
Baron Kelvin, also known as Lord Kelvin (1824-1907)
• 0 K = -273.15°C = -459.67°F
• Absolute zero is defined as the coldest possible
temperature
– Near this temperature, all molecular motion ceases
– This would have serious quantum mechanical consequences
• The average temperature of the
universe today is only ~2.73K
• Currently, absolute zero cannot be
achieved.
– World record set in 1999: 0.0000000001
(or 1.0x10-10) K.