Basic Foundations SI Units

The table below lists the base SI units for different measurements: 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 Prefixes are used to allow convenient measurement over vastly different scales

Mass vs. Weight Mass measures the amount of matter an object contains 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 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…) 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 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 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 Atomic mass units (amu) is used to express the masses of atoms 1 amu = x grams Note: 1/ x = x = N A where N A is called Avogadro’s number

Temperature Celsius (°C) Freezing point of water at 1 atm: 0°C Freezing point of water at 1 atm: 0°C Boiling point of water at 1 atm: 100°C Boiling point of water at 1 atm: 100°C Kelvin (K) “Absolute scale” “Absolute scale” Lowest possible temperature: 0K Lowest possible temperature: 0K K = °C Fahrenheit is the usual temperature scale in America. Fahrenheit is the usual temperature scale in America. The freezing point of water at 1 atm is 32°F The freezing point of water at 1 atm is 32°F The boiling point of water at 1 atm is 212°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 Many times it is necessary to combine SI units to make other useful units Volume may be derived by length: V = (length) 3 Example: cm 3 Or one may use liters (L) 1 dm 3 = 1 L 1 cm 3 = 1 mL These relationships must be memorized

SI-Derived Units Derived units may also combine with other SI or 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 Density is a common measurement which gives the ratio of mass to volume of an object – The SI-derived unit is given as kg/m 3 Density may be seen in units: Density may be seen in units: – g/cm 3 (solids) – g/mL (liquids) – g/L (gases)

SI-Derived Units SI-Derived units come up frequently in science, and may be more convenient 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· m 2 · s -2 These derived units make expressing measurements easier These derived units make expressing measurements easier

Non-SI Units Non-SI Units may also be convenient Non-SI Units may also be convenient The SI unit of pressure is the Pascal The SI unit of pressure is the Pascal – 1 N/m 2 Other units are common: Other units are common: – Atm, bar, psi, Torr, mm Hg 1 atm = Pa = bar 1 atm = 14.7 psi (pounds per square inch) 1 atm = 760 Torr = 760 mm Hg

Chemistry Coffee Break SI unit of temperature is the Kelvin SI unit of temperature is the Kelvin The Kelvin scale is named after William Thomson, 1 st Baron Kelvin, also known as Lord Kelvin ( ) The Kelvin scale is named after William Thomson, 1 st Baron Kelvin, also known as Lord Kelvin ( ) 0 K = °C = °F Absolute zero is defined as the coldest possible temperature Absolute zero is defined as the coldest possible temperature – Near this temperature, all molecular motion ceases – This would have serious quantum mechanical consequences Finding Absolute Zero The average temperature of the universe today is only ~2.73K The average temperature of the universe today is only ~2.73K Currently, absolute zero cannot be achieved. Currently, absolute zero cannot be achieved. – World record set in 1999: (or 1.0x ) K.