It provides a specific measure of the number of atoms or molecules in a sample of matter. The mole is an amount unit similar to familiar units like pair, dozen, gross, etc. This experimental approach required the introduction of a new unit for amount of substances, the mole, which remains indispensable in modern chemical science. Today, sophisticated instruments allow the direct measurement of these defining microscopic traits however, the same traits were originally derived from the measurement of macroscopic properties (the masses and volumes of bulk quantities of matter) using relatively simple tools (balances and volumetric glassware). However, because a hydrogen peroxide molecule contains two oxygen atoms, as opposed to the water molecule, which has only one, the two substances exhibit very different properties. For example, water, H 2O, and hydrogen peroxide, H 2O 2, are alike in that their respective molecules are composed of hydrogen and oxygen atoms. The identity of a substance is defined not only by the types of atoms or ions it contains, but by the quantity of each type of atom or ion. The few exceptions to this guideline are very light ions derived from elements with precisely known atomic masses.

Even when calculating the mass of an isolated ion, the missing or additional electrons can generally be ignored, since their contribution to the overall mass is negligible, reflected only in the nonsignificant digits that will be lost when the computed mass is properly rounded. Moreover, the mass of an electron is negligibly small with respect to the mass of a typical atom. Even though a sodium cation has a slightly smaller mass than a sodium atom (since it is missing an electron), this difference will be offset by the fact that a chloride anion is slightly more massive than a chloride atom (due to the extra electron). This approach is perfectly acceptable when computing the formula mass of an ionic compound. Note that the average masses of neutral sodium and chlorine atoms were used in this computation, rather than the masses for sodium cations and chlorine anions. Figure 3.2 outlines the calculations used to derive the molecular mass of chloroform, which is 119.37 amu.įigure 3.4 Table salt, NaCl, contains an array of sodium and chloride ions combined in a 1:1 ratio. The average molecular mass of a chloroform molecule is therefore equal to the sum of the average atomic masses of these atoms. The molecular formula of chloroform indicates that a single molecule contains one carbon atom, one hydrogen atom, and three chlorine atoms. Consider chloroform (CHCl 3), a covalent compound once used as a surgical anesthetic and now primarily used in the production of tetrafluoroethylene, the building block for the 'anti-stick' polymer, Teflon. Formula Mass for Covalent Substancesįor covalent substances, the formula represents the numbers and types of atoms composing a single molecule of the substance therefore, the formula mass may be correctly referred to as a molecular mass. These ideas can be extended to calculate the formula mass of a substance by summing the average atomic masses of all the atoms represented in the substance’s formula. Formula MassĪn earlier chapter of this text described the development of the atomic mass unit, the concept of average atomic masses, and the use of chemical formulas to represent the elemental makeup of substances.

Understanding the relationship between the masses of atoms and the chemical formulas of compounds allows us to quantitatively describe the composition of substances. For example, Dalton’s atomic theory was an attempt to explain the results of measurements that allowed him to calculate the relative masses of elements combined in various compounds. Many argue that modern chemical science began when scientists started exploring the quantitative as well as the qualitative aspects of chemistry. Explain the relation between mass, moles, and numbers of atoms or molecules, and perform calculations deriving these quantities from one another.Define the amount unit mole and the related quantity Avogadro’s number.Calculate formula masses for covalent and ionic compounds.By the end of this section, you will be able to:

Molar mass in periodic table