Units of Measurement

A measurement usually consists of a unit and a number expressing the quantity of that unit. We may express the same physical measurement with different units, which can create confusion. For example, the mass of a sample weighing 1.5 g also may be written as 0.0033 lb or 0.053 oz. To ensure consistency, and to avoid problems, scientists use a common set of fundamental units, several of which are listed in Table 1.1 . These units are called SI units after the Système International d’Unités.

We define other measurements using these fundamental SI units. For example, we measure the quantity of heat produced during a chemical reaction in joules, (J), where

Table 1.2 provides a list of some important derived SI units, as well as a few common non-SI units.

^† The mass of the international prototype changes at a rate of approximately 1 mg per year due to reversible surface contamination. The reference mass, therefore, is determined immediately after its cleaning by a specified procedure.

Chemists frequently work with measurements that are very large or very small. A mole contains 602 213 670 000 000 000 000 000 particles and some analytical techniques can detect as little as 0.000 000 000 000 001 g of a compound. For simplicity, we express these measurements using scientific notation; thus, a mole contains 6.022 136 7 × 10²³ particles, and the detected mass is 1 × 10^–15 g. Sometimes it is preferable to express measurements without the exponential term, replacing it with a prefix (Table 2.3). A mass of 1×10^–15 g, for example, is the same as 1 fg, or femtogram.