The molar mass of an element (or compound) is the mass in grams of 1 mole of that substance, a property expressed in units of grams per mole (g/mol) (see ).Įach sample contains 6.02 \(×\) 10 23 molecules or formula units-1.00 mol of the compound or element. The masses of 1 mole of different elements, however, are different, since the masses of the individual atoms are drastically different. This constant is properly reported with an explicit unit of “per mole,” a conveniently rounded version being 6.022 \(×\) 10 23/mol.Ĭonsistent with its definition as an amount unit, 1 mole of any element contains the same number of atoms as 1 mole of any other element. The number of entities composing a mole has been experimentally determined to be 6.02214179 \(×\) 10 23, a fundamental constant named Avogadro’s number ( N A) or the Avogadro constant in honor of Italian scientist Amedeo Avogadro. The mole provides a link between an easily measured macroscopic property, bulk mass, and an extremely important fundamental property, number of atoms, molecules, and so forth. One Latin connotation for the word “mole” is “large mass” or “bulk,” which is consistent with its use as the name for this unit. A mole is defined as the amount of substance containing the same number of discrete entities (such as atoms, molecules, and ions) as the number of atoms in a sample of pure 12C weighing exactly 12 g. It provides a specific measure of the number of atoms or molecules in a bulk 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, we possess sophisticated instruments that 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. Table salt, NaCl, contains an array of sodium and chloride ions combined in a 1:1 ratio.
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