Molar Mass and Chemical Compounds

In Section 9.2, you learned how to calculate the number of atoms—expressed in moles of atoms—in a sample of an element. Molecular substances are composed of molecules, and in this section you will learn how to calculate the number of molecules, expressed in moles of molecules, in a sample of a molecular substance. Remember that, like dozen, the collective unit mole can be used to describe the numbet of anything. There are 6.022 X 10 atoms in a mole of carbon atoms, there are 6.022 X 10 electrons in a mole of electrons, and there are 6.022 x 1023 H2O molecules in a mole of water. 

Molecular Mass and Molar Mass of Molecular Compounds 

Molecular mass = the sum of the atomic masses of each atom in the molecule 

Therefore, the molecular mass of water, H2O, is equal to the sum of the atomic masses of two hydrogen atoms and one oxygen atom, which can be found on the periodic table. 

Note that the atomic mass of each element is multiplied by the number of atoms of that element in a molecule of the compound. 

---

C05-0021.A 2.96-g sample of a compound of mercury and chlorine is vaporized in a 1.000-L bulb at 307 °C, and the final pressure is found to be 394 torr. What are the molar mass and chemical formula of the compound ... 

Contrary to the usual organic compounds, polymers are far from being homogeneuos maferials (i.e., polymer chains do not possess the same molar mass and chemical structure). As matter of fact, many synthetic polymers are heterogeneous in several respects. Homopolymers may exhibit both molar-mass distribution (MMD) and end-groups (EG) distribution. Copolymers may also show chemical composition distribution (CCD) and functionality distribution (FTD) in addition to the MMD. Therefore, different kinds of heterogeneity need to be investigated in order to proceed to the structural and molecular characterization of polymeric materials. 

As emphasized in Section 2-, many of the calculations in chemistry involve converting back and forth among the mass of a substance, the number of moles, and the number of atoms and/or molecules. These calculations are all centered on the mole. The connections shown in Figure apply to chemical compounds as well as to atoms of pure elements. Molar mass and Avogadro s number provide links between mass of a sample, the number of moles, and the number of molecules. 

The role of specific chemical groups in or on a polymer chain in radiation degradation can be usefully investigated using model compounds of low molar mass. Much valuable information can be deduced in this way, but the unique properties of polymers are a consequence of their high molar mass and this can also apply to radiation degradation, thereby limiting conclusions drawn from radiation studies on model compounds of low molar mass. 

In this chapter, you will learn about the relationships between chemical formulas, molar masses, and the masses of elements in compounds. 

The molar mass of a compound is equal to the molar mass of the empirical formula times a whole number, n. There are several experimental techniques for finding the molar mass of a molecular compound even though the compound s chemical composition and formula are unknown. If you divide the experimental molar mass by the molar mass of the empirical formula, you can figure out the value of n needed to scale the empirical formula up to give the molecular formula. 

The graphing calculator can run a program that calculates the molar mass of a compound given the chemical formula for the compound. This program will prompt for the number of elements in the formula, the number of atoms of each element in the formula, and the atomic mass of each element in the formula. It then can be used to find the molar masses of various compounds. 

The mass of your backpack is the sum of the mass of the pack plus the masses of the books, notebooks, pencils, lunch, and miscellaneous items you put into it. You could find its mass by determining the mass of each item separately and adding them together. Similarly, the mass of a mole of a compound equals the sum of the masses of every particle that makes up the compound. You know how to use the molar mass of an element as a conversion factor in calculations. You also know that a chemical formula indicates the number of moles of each element in a compound. With this information, you can now determine the molar mass of a compound. 

You are given only the chemical formula. Assume you have one mole of NaHCOj. Calculate the molar mass and the mass of each element in one mole to determine the percent by mass of each element in the compound. The sum of all percents should be 100%. 

The chemical formula of a compound can be determined if the molar mass and the empirical formula are known. 

Converting Moles of Compounds Solving mass-mole-number problems involving compounds requires a very similar approach to the one for elements. We need the chemical formula to find the molar mass and to determine the moles of a given element in the compound. These relationships are shown in Figure 3.4, and an example is worked through in Sample Problem 3.2. 

Avogadro s number, molar mass, and the chemical formula of a compound are three useful conversion factors. What unit conversions can be accomplished using these conversion factors ... 

The molar mass of a compound can be calculated from its chemical formula and can be used to convert from mass to moles of that compound. 

Conversions between mass, moles, and the number of particles are summarized in Figure 10.11. Note that molar mass and the inverse of molar mass are conversion factors between mass and number of moles. Avogadros number and its inverse are the conversion factors between moles and the number of representative particles. To convert between moles and the number of moles of atoms or ions contained in the compound, use the ratio of moles of atoms or ions to 1 mole of compound or its inverse, which are shown on the upward and downward arrows in Figure 10.11. These ratios are derived from the subscripts in the chemical formula. 

The worldwide investigation of new silicon-containing polymers as innovative high-tech materials requires the development of modem analytical techniques. The methods should allow the characterization of the starting compounds as well as a comprehensive analysis of the main and by-products with regard to their molar masses and molar mass distribution. Precise information on the distribution of the chemical constituents and functional groups is also needed in order to understand polymer formation processes, degradation behavior, and stmcture-property relationships. 

Thinking it Through This type of question starts with a chemical formula and uses it to determine the mass percent for any element in a compound. Mass percent is a comparison of the mass of the element being considered to the molar mass of the compound. The ratio is then multiplied by 100 to make it a percent, which is the same as parts per hundred. In this case, the mass of oxygen present must be compared to the molar mass of the compound,... 

Another problem with GPC of condensation resins is the calibration of the columns. Because in the ohgomeric and polymeric regions of the resins no compounds with a special and singular molar mass and a clear molecular structure are available, similar or chemically related substances have to be used as calibration standards. However, differences in the hydrodynamic volumes even at the same molar mass cannot be excluded totally. This uncertain cahbration of the columns also induces a great uncertainty in the calculation of molar mass averages on the basis of the chromatograms obtained. 

The surface of any material governs its interactions with the environment. Knowledge over and control of these interaction is especially important when a material is in contact with the biosystem, for example, when applied as transplant, in tissue engineering, in cell cultures, and in blood contact, as weU as in biosensors in medicinal diagnosis, fluids analysis, environmental moititoring, and many other areas. Whereas, on the one hand, the bulk properties of the material are essential for its successful application, for example, as a catheter or a heart valve, special attention has to be paid to render to the surface suitable biocompatible or bioactive properties, no matter of the chemical composition of the bulk material. This is usually achieved by any surface modification process by low molar mass or polymeric compounds. An essential feature of such a modification procedure is the need for a permanent and bioresistant surface finish [87]. 

Knowing the chemical formula and the molecular mass of a compound enables us to calculate the percent composition by mass—the percent by mass of each element in a compound. It is useful to know the percent composition by mass if, for example, we needed to verify the purity of a compound for use in a laboratory experiment. From the formula we could calculate what percent of the total mass of the compound is contributed by each element. Then, by comparing the result to the percent composition obtained experimentally for our sample, we could determine the purity of the sample. Mathematically, the percent composition is obtained by dividing the mass of each element in 1 mole of the compound by the molar mass of the compound and multiplying by 100 percent ... 

Two of the more recently developed detectors, namely, the Hall electrolytic conductivity detector (ELCD) and the photoionization detector (PID) are recommended by the EPA for the analysis of volatile and semivolatile halogenated organic compounds and low molar mass aromatics. Chemical emission based detectors, such as the thermal energy analyzer (TEA) for its determination of... 

Percent Composition of a Compound The makeup of a compound is most conveniently expressed in terms of its percent composition, which is the percent by mass of each element the compound contains. A knowledge of its chemical formula enables us to calculate the percent composition. Experimental determination of percent composition and the molar mass of a compound enables us to determine its chemical formula. 

---