Recording-2025-03-09T08:22:26.518Z

1.5 Units of Measurement

• Many properties of matter are quantitative, associated with numbers that must have specified units.

• Example: Length of a pencil: saying 17.5 is meaningless; must specify as 17.5 centimeters (cm).

Metric System

• The metric system was developed in France during the late 18th century and is used in most countries.

• Units of measurement: important for scientific measurement.

• SI units: established in 1960, known as "Système International d'Unites."

  • Seven base units from which all other units are derived (Table 1.3).

Base Units

• Length: Meter (m)

• Mass: Kilogram (kg)

• Temperature: Kelvin (K)

Prefixes in SI Units

• Prefixes indicate decimal fractions or multiples of units.

• Example: "milli" represents a 10^-3 fraction (1/1000).

  • Milligram (mg) = 10^-3 grams

  • Millimeter (mm) = 10^-3 meters

• Table 1.4 presents common prefixes used in chemistry.

Understanding Non-SI Units

• Some non-SI units are still used by scientists; SI unit equivalents will be given.

• Conversions from non-SI to SI units discussed in Section 1.7.

Length and Mass

• SI base unit of length (meter) is slightly longer than a yard.

• SI base unit of mass is kilogram (kg), unusual for using the prefix "kilo".

Example Exercises

• Example: What is the name of a unit that equals:

  • a) 10^-9 grams? Answer: Nanogram (ng)

  • b) 10^-6 seconds? Answer: Microsecond (µs)

  • c) 10^-3 meters? Answer: Millimeter (mm)

Temperature Measurement

• Temperature is a measure of hotness/coldness; dictates direction of heat flow.

• Common scales: Celsius and Kelvin.

  • Celsius: 0°C (freezing point of water), 100°C (boiling point).

  • Absolute zero: 0 K (where all thermal motion ceases) equals -273.15°C.

• Relationships: K = °C + 273.15.

  • Both Celsius and Kelvin scales have equal-sized units.

Derived SI Units

• Derived units obtained by multiplying/dividing the base units.

• Example: Speed = Distance/Time → meters per second (m/s).

• Common derived units in chemistry: Volume and Density.

Volume Measurement

• Volume of a cube = Length³ ⇒ Derived SI unit: cubic meter (m³).

• Smaller volume units: cubic centimeters (cm³), often used in chemistry.

  • Liter (L) = Cubic decimeter (dm³); 1 L = 1000 mL.

  • 1 mL = 1 cm³.

Density Defined

• Density: mass per unit volume; formula: Density = Mass/Volume.

• Common units: grams per cubic centimeter (g/cm³) or grams per milliliter (g/mL).

  • Example: Density of water = 1.00 g/mL at 25°C.

• Densities are temperature-dependent; typically reported at 25°C.

Common Densities (Table 1.5)

• Air: 0.001 g/cm³

• Wood (Balsa): 0.16 g/cm³

• Ethanol: 0.79 g/cm³

• Water: 1.00 g/cm³

• Ethylene Glycol: 1.09 g/cm³

• Table Sugar: 1.59 g/cm³

• Table Salt: 2.16 g/cm³

Common Confusions

• Density vs. Weight:

  • Higher density means mass per unit volume is greater.

  • 1 kg of air vs. 1 kg of iron: same mass, different densities.

Units of Energy

• SI unit for energy: Joule (J).

• Derived unit: 1 J = 1 kg·m²/s²; honors scientist James Joule (1818-1889).

• Example: 2 kg mass at 1 m/s velocity has kinetic energy of 1 J.

Sample Exercises

• Example: Calculate the density of mercury with given quantities.