1.5 SI Units

SI Units

Metric system is the oldest method of measurement. In each type of measurement, there is a base unit. The other units are related to the base unit by power of 10. These are called metric prefixes. The prefix of the unit name indicates if the unit is larger or smaller than the base unit.

Length is measured by determining the distance between two points. Mass is determined by the amount of matter in an object. Volume is determined by the amount of space occupied by a three -dimensional object.

Metric Base Units

The metric system uses the following base units:

Find below common metric prefixes.

Table 1.3.

All reported measurements must include an appropriate unit of measurement because to say that a substance has “a mass of 10,” for example, does not tell whether the mass was measured in grams, pounds, tons, or some other unit.  To establish worldwide standards for the consistent measurement of important physical and chemical properties, an international body called the General Conference on Weights and Measures devised the Système internationale d’unités (or SI). The International System of Units is based on metric units and requires that measurements be expressed in decimal form. Table 1.7 “SI Base Units” lists the seven base units of the SI system; all other SI units of measurement are derived from them.

By attaching prefixes to the base unit, the magnitude of the unit is indicated; each prefix indicates that the base unit is multiplied by a specified power of 10. The prefixes, their symbols, and their numerical significance are given below.  To study chemistry, you need to know the information presented in tables 1.3 and 1.4 containing the SI base units and the prefixes to be used with the SI units, respectively.   

Table 1.4 Prefixes used with SI units of mass, volume, and length

The units of measurement you will encounter most frequently in chemistry are those for mass, volume, and length.  The basic SI unit for mass is the kilogram (kg), but in the laboratory, mass is usually expressed in either grams (g) or milligrams (mg): 1000 g = 1 kg, 1000 mg = 1 g, and 1,000,000 mg = 1 kg.  Units for volume are derived from the cube of the SI unit for length, which is the meter (m). Thus, the basic SI unit for volume is cubic meters (length × width × height = m³).  In chemistry, however, volumes are usually reported in cubic centimeters (cm³) and cubic decimeters (dm³) or milliliters (mL) and liters (L), although the liter is not an SI unit of measurement. The relationships between these units are as follows:

1 L = 1000 mL = 1 dm³1 mL = 1 cm³1000 cm³ = 1 L

Liter: It is a nonstandard metric unit. Liter is defined as volume of 0.1 m cube or 1 decimeter cube. A smaller non SI unit is often used to measure volume which is 1 milliliter. Since 1000ml = 1L and 1 L contains 1000 cm3, therefore 1 cm3= 1ml. Usually 1 cm3 is used for solid substances and 1 ml is used liquid substances.

Figure 1.20 Definition of 1 Liter

Table 1.5 SI Units & Symbols

***The new definition of the kilogram sets it equal to the mass of 1.4755214 x 10⁴⁰ photons from a cesium atom.

Kelvin is defined in terms of Boltzman constant

Mol is defined in terms of Avogadro number 6.02214076 × 10²³ mol⁻¹.