Sulfur hexafluoride is used as a model compound for sarin. The nerve toxin Sarin (molecular formula C 4H 10FPO 2). Scientists and engineers who study the spread of chemical weapons such as sarin (yes, there are such scientists) would like to have a less dangerous chemical, indeed one that is nontoxic, so they are not at risk themselves. It is regarded as one of the most deadly toxins known, estimated to be about 500 times more potent than cyanide. Sarin (molecular formula C 4H 10FPO 2) is a nerve toxin that was first synthesized in 1938 (Figure 4). On March 20, 1995, the Japanese terrorist group Aum Shinrikyo (Sanskrit for “Supreme Truth”) released some sarin gas in the Tokyo subway system twelve people were killed, and thousands were injured. The few exceptions to this guideline are very light ions derived from elements with precisely known atomic masses.Ĭhemistry Is Everywhere: Sulfur Hexafluoride 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. = 58.4425 amu = the molecular mass of NaClįigure 3. The formula mass for this compound is computed as 58.44 amu (Figure 3). Sodium chloride is an ionic compound composed of sodium cations, Na +, and chloride anions, Cl −, combined in a 1:1 ratio. Keep in mind, however, that the formula for an ionic compound does not represent the composition of a discrete molecule, so it may not correctly be referred to as the “molecular mass.”Īs an example, consider sodium chloride, NaCl, the chemical name for common table salt. The formula mass for an ionic compound is calculated in the same way as the formula mass for covalent compounds: by summing the average atomic masses of all the atoms in the compound’s formula. Ionic compounds are composed of discrete cations and anions combined in ratios to yield electrically neutral bulk matter. What is the molecular mass of each substance?Ī) 64.065 amu b) 87.969 amu Formula Mass for Ionic Compounds The compound C 2H 6 also has a common name-ethane. = 30.070 amu = the molecular mass of C 2H 6 Figure 1 outlines the calculations used to derive the molecular mass of chloroform, which is 119.377 amu. 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 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. In an earlier chapter, we 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. We can argue that modern chemical science began when scientists started exploring the quantitative as well as the qualitative aspects of chemistry. Calculate formula masses for covalent and ionic compounds.By the end of this section, you will be able to:
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