Units, Measurement, and Problem Solving: Foundations of General Chemistry

Units of Measurement

Introduction to Units of Measurement

In chemistry, precise calculations are essential, but the most critical aspect of any numerical answer is the unit of measure. Units provide a standardized way to express and compare quantities, ensuring clarity and consistency in scientific communication.

• Unit: A fixed, agreed-upon quantity used as a reference for measurement (e.g., pounds, inches).

• Measurement: A number associated with a unit (e.g., 10 inches).

International System of Units (SI Units)

To minimize confusion, scientists worldwide use the SI units (Système International d’Unités), which are based on the metric system. Each SI unit is a combination of a base unit and, when needed, a prefix multiplier.

• Base Unit: The fundamental unit for a physical quantity (e.g., meter for length).

• Prefix Multiplier: A factor that scales the base unit (e.g., kilo-, milli-).

Scientific Units of Measurement

There are seven SI base units from which all other units are derived. Each physical quantity has a specific base unit.

Quantity	Unit	Symbol
Length	Meter	m
Mass	Kilogram	kg
Time	Second	s
Temperature	Kelvin	K
Amount of substance	Mole	mol
Electric current	Ampere	A
Luminous intensity	Candela	cd

Basic SI Units

The three most commonly used SI base units in chemistry are:

• Length: Meter (m)

• Mass: Kilogram (kg)

• Time: Second (s)

Length: Meter (m)

• Defined (since 1983) as the distance light travels in $1/299,792,458$ seconds.

• Examples: Human height ≈ 2 m; Dust particle ≈ 0.0001 m.

• Conversion: $1\ \text{meter} = 39.4\ \text{inches} = 1.094\ \text{yards}$

Mass: Kilogram (kg)

• The SI base unit for mass.

• Mass is the quantity of matter in an object, distinct from weight (which is the force due to gravity).

• Conversion: $1\ \text{kg} = 2.2\ \text{pounds}$

Time: Second (s)

• The SI base unit for time.

• Originally defined as $1/60$ of a minute; now defined by atomic standards (cesium clock).

Example: Measuring the trajectory of a spacecraft requires precise units to avoid costly errors.

Additional info: The full set of SI base units also includes temperature (Kelvin), amount of substance (mole), electric current (ampere), and luminous intensity (candela), which are essential for various branches of chemistry and physics.