SI Units and Measurements: Quick Reference

Measurements and SI overview

  • Measurements have two parts: a number and a unit.
  • We will be given measurements to use in calculations; actual measurements aren’t performed in this lecture.
  • We’ll start with units, then examine mass and weight, then volume.

SI base units

  • SI stands for International System of Units; base on the metric system and more fine-tuned.
  • Base units (illustrative):
    • Length: m (meter)
    • Time: s (second)
    • Mass: kg (kilogram)
    • Temperature: K (kelvin)
    • Amount of substance: mol (mole)
  • Electric current (A) is a base unit but not covered until later in General Chemistry II.
  • Temperature scales to know: Kelvin (K); Celsius and Fahrenheit discussed later.
  • The mole abbreviation is mol; do not abbreviate as m (m is meter).
  • Pandela is not a unit of measure used in General Chemistry.
  • Volume is not a base unit; it is derived.

SI prefixes

  • You must memorize the prefix word, its symbol, and its meaning (10^n).
  • Common prefixes (most frequently used; marked with asterisks in many courses):
    • milli (m) — 10^{-3}
    • kilo (k) — 10^{3}
    • micro (μ) — 10^{-6}
  • Example relationships (consistent pattern):
    • 1mm=10−3m1\text{mm} = 10^{-3}\text{m}
    • 1km=103m1\text{km} = 10^{3}\text{m}
    • 1μm=10−6m1\text{μm} = 10^{-6}\text{m}
  • Other prefixes to know (less frequently used): tera (T, 10^{12}), giga (G, 10^{9}), mega (M, 10^{6}), centi (c, 10^{-2}), deci (d, 10^{-1}), nano (n, 10^{-9}), pico (p, 10^{-12}).

Mass and Weight

  • The SI unit for mass is the kilogram (kg).
  • Mass vs weight:
    • Mass = amount of matter; mass is invariant with location.
    • Weight = gravity acting on mass; weight varies with gravity.
  • Mass is measured with a balance; weight is measured with a spring.
  • Relationship between mass and weight is defined by gravity; on the Moon, weight is about a tenth of Earth weight (per the lecture’s example).
  • On Earth, a mass has a defined weight via gravity; mass remains the same on the Moon, but weight decreases.

Volume and Derived Units

  • Volume is a derived unit: V=l⋅w⋅hV = l \cdot w \cdot h
  • Base length unit is the meter, so volume unit is m3m^3 (but we rarely use cubic meters in chemistry).
  • Commonly used volume units in chemistry: liter (L), milliliter (mL), and cubic centimeter (cm^3).
  • Conversions:
    • 1m3=1000L1\text{m}^3 = 1000\text{L}
    • 1L=1000mL1\text{L} = 1000\text{mL}
    • 1cm3=1mL1\text{cm}^3 = 1\text{mL}
    • 1cm=10−2m1\text{cm} = 10^{-2}\text{m}
    • Therefore, 1cm3=(10−2)3m3=10−6m31\text{cm}^3 = (10^{-2})^3\text{m}^3 = 10^{-6}\text{m}^3
  • Memorized note: 1 mL = 1 cm^3; 1 L = 1000 mL.
  • Sugar cube = roughly 1 cm^3.
  • Because of convenience, we use L, mL, and cm^3 more than m^3 in chemistry.
  • Blue-box memory item (high-importance): 1mL=1cm31\text{mL} = 1\text{cm}^3 and 1L=1000mL1\text{L} = 1000\text{mL}.