Sodium chloride formula unit

Molar mass is important when we need to know the number of atoms in a sample. It would be impossible to count out 6 X ID23 atoms of an element, but it is very easy to measure out a mass equal to the molar mass of the element in grams. Each of the samples shown in Fig. E.2 was obtained in this way each sample contains the same number of atoms of the element (6.022 X 1023), but the masses vary because the masses of the atoms are different (Fig. E.4). The same rule applies to compounds. Flence, if we measure out 58.44 g of sodium chloride, we obtain a sample that contains 1.000 mol NaCl formula units (Fig. E.5). 

FIGURE E.5 Each sample contains 1 mol of formula units of an ionic compound. From left to right are 58 g of sodium chloride (NaCl), 100 g of calcium carbonate (CaCO,), 278 g of iron(ll) sulfate heptahydrate (FeS04-7H.0), and 78 g of sodium peroxide (Na. O,). 

In an electrolyte solution, each formula unit contributes two or more ions. Sodium chloride, for instance, dissolves to give Na+ and Cl ions, and both kinds of ions contribute to the depression of the freezing point. The cations and anions contribute nearly independently in very dilute solutions, and so the total solute molality is twice the molality of NaCl formula units. In place of Eq. 5a we write... 

The equation states that elementary sodium reacts with elementary chlorine to produce sodium chloride, table salt. (The fact that chlorine is one of the seven elements that occur in diatomic molecules when not combined with other elements is indicated.) The numbers before the Na and NaCI are coefficients, stating how many formula units of these substances are involved. If there is no coefficient in a balanced equation, a coefficient of 1 is implied, and so the absence of a coefficient before the Cl2 implies one Cl2 molecule. The equation thus states that when the two reagents react, they do so in a ratio of two atoms of sodium to one molecule of chlorine, to form two formula units of sodium chloride. In addition, it states that when the two reagents react, they do so in a ratio of 2 mol of sodium to 1 mol of chlorine molecules, to form 2 mol of sodium chloride. The ratios of moles of each reactant and product to every other reactant or product are implied ... 

The method can be illustrated by reference to a classical 1933 study of the defects present in wilstite, iron monoxide. Wustite adopts the sodium chloride (NaCl) structure, and the unit cell should contain 4 Fe and 4 O atoms in the unit cell, with an ideal composition FeOi.o, but in reality the composition is oxygen rich and the unit cell dimensions also vary with composition (Table 1.1). Because there is more oxygen present than iron, the real composition can be obtained by assuming either that there are extra oxygen atoms in the unit cell (interstitial defects) to give a composition FcO 1 +v, or that there are iron vacancies present, to give a formula Fci-JD. It is... 

Pure potassium bromide, KBr, which adopts the sodium chloride structure, has the fraction of empty cation sites due to Schottky defects, ncv/Nc, equal to 9.159xl0-21 at 20°C. (a) Estimate the enthalpy of formation of a Schottky defect, Ahs. (b) Calculate the number of anion vacancies per cubic meter of KBr at 730°C (just below the melting point of KBr). The unit cell of KBr is cubic with edge length a = 0.6600 nm and contains four formula units of KBr. 

We next insert concentration terms, noting that one sodium ion and one chloride are formed per formula unit of sodium chloride (which is why we call it a 1 1 electrolyte). Accordingly, the concentrations of the two ions, [Na+] and [Cl-], are the same as [NaCl], so... 

For nonmolecular substances like sodium chloride, the use of the word mole, with its connotation of molecules, is inappropriate. A comparable unit, the gram formula weight, is used it is defined as the sum in grams of the atomic weights of all the atoms in the chemical formula of the substance. For sodium chloride (NaCl), one gram formula weight is calculated as... 

The sodium chloride unit cell contains four formula units of NaCl. If you find this difficult to see, work it out for yourself by counting the numbers of ions in the different sites and applying the information given in Table 1.4. 

The contents of the unit cell of any compound must contain an integral number of formula units. (Why ) Note that unit cell boundaries slice" atoms into fragments An atom on a face Will be split in half between two cells one on an edge will be splu into gunners among Jour cells, etc Identify the number of Na and Cl ions in the unit cell of sodium chloride illustrated in Fig. 4.1a and state how many formula units of NaCl the unit cell contains. Give a complete analysis. 

We do not speak of a molecule of an ionic compound. However, it is useful to be able to refer to a representative group of ions with the number of atoms given by the formula. This group of ions- is called a formula unit. For example, the formula unit of sodium chloride, NaCl, consists of one Na+ ion and one Cl ion and the formula unit of ammonium sulfate, (NH4)2S04, consists of two NH4+ ions and one S042- ion. 

NaCI represents the compound containing one atom of sodium and one atom of chlorine. The masses on these elements are too small to determine individually or as individual ionic units. However, using the mole we can determine the mass of the compound represented by the formula NaCI. One mole of sodium atoms has a mass of 23.0 grams and one mole of chlorine atoms has a mass of 35.5 grams. Therefore, one mole of sodium chloride molecules has a mass of 58.5 grams. 

Except where the context requires otherwise, the term refers to a set of -> molecular entities containing isotopes in their natural abundance. The wording of the definition given in the first paragraph is intended to embrace both cases, such as graphite, sodium chloride, or a surface oxide, where the basic structural units may not be capable of isolated existence, as well as those cases where they are. In common chemical usage, generic and specific chemical names (such as -> radical or hydroxide ion) or chemical formulae refer either to a chemical species or to a molecular entity. 

Although skeleton equations are useful, they do not fully describe chemical reactions. To understand why, consider the skeleton equation showing the formation of sodium chloride (above). According to this equation, one sodium atom reacts with one chlorine molecule containing two chlorine atoms. The product is one formula unit of sodium chloride, containing one atom of sodium and one atom of chlorine. Where has the extra chlorine atom gone ... 

For example, one mole of carbon contains 6.02 x 1023 atoms of C. One mole of sodium chloride contains 6.02 x 1023 formula units of NaCl. 

Any specimen of matter of macroscopic size, that is, big enough to be seen by the eye, must be essentially electrically neutral. It might have an excess of either positive or negative ions, and thus be charged positively or negatively, but the amount of charge, measured in units e, is always small compared with the number of atoms. Hence a crystal of sodium chloride must contain substantially as many Na+ ions as Cl ions, and its formula must be Na+Cl . The composition of the crystal and the formula of the compound are thus determined by the ionic valences of the constituent elements these ionic valences must add up to zero. 

The number in front of the formula for water and also in front of the formula for table salt indicates the number of units of each that enter into the reaction. Two water molecules react with two units of sodium chloride to form one unit of sodium hydroxide, one molecule of chlorine gas, Clj, and one molecule of hydrogen gas, Hj. In other words, if we were to write each reactant individually, the reaction would be... 

Recall from Section 8.2 that a sample of an ionic compound contains crystals formed from many ions arranged in a pattern. Because no single particle of an ionic compound exists, ionic compounds are represented by a formula that provides the simplest ratio of the ions involved. The simplest ratio of the ions represented in an ionic compound is called a formula unit. For example, the formula KBr represents a formula unit for potassium bromide because potassium and bromide ions are in a one-to-one ratio in the compound. A formula unit of magnesium chloride is MgCl2 because two chloride ions exist for each magnesium ion in the compound. In the compound sodium phosphide, three sodium ions exist for every phosphide ion. What is the formula unit for sodium phosphide ... 

Suppose you need to determine the formula for one formula unit of the compound that contains sodium and chloride ions. Write the symbol and charge for each ion. 

The ratio of ions must be such that the number of electrons lost by the metal is equal to the number of electrons gained by the nonmetal. Because the sum of the oxidation numbers of these ions is zero, these ions must be present in a one-to-one ratio. One sodium ion transfers one electron to one chloride ion, and the formula unit is NaCl. 

Avogadro s number is an enormous number, as it must be in order to count extremely small particles. As you can imagine, Avogadro s number would not be convenient for measuring a quantity of marbles. Avogadro s number of marbles would cover the surface of Earth to a depth of more than six kilometers But you can see in Figure 11-2 that it is convenient to use the mole to measure substances. One-mole quantities of three substances are shown, each with a different representative particle. The representative particle in a mole of water is the water molecule, the representative particle in a mole of copper is the copper atom, and the representative particle in a mole of sodium chloride is the formula unit. 

What mass of sodium chloride contains 4.59 x lO formula units ... 

Two atoms of monatomic sodium react with one molecule of diatomic chlorine to form two formula units of sodium chloride. 

Names for stoichiometric phases, such as NaCl, are derived simply as in Chapter IR-5, whereas formulae are derived as presented in Chapter IR-4. Although NaCl in the solid state consists of an infinite network of units, (NaCl)oo, the compound is named sodium chloride and represented symbolically as NaCl. 

---