Molar mass molecular formula determination from

The empirical formula CH2 is not a stable substance. It is necessary to determine the molar mass to determine the molecular formula. If this hydrocarbon were a gas or an easily volatilized liquid, its molar mass could be determined from the density of the gas, as shown in Chapter 5. Supposing such a determination yields a molar mass of about 55 g/mol, what is the molecular formula ... 

Formulas describe the composition of compounds. Empirical formulas give the mole ratio of the various elements. However, sometimes different compounds have the same ratio of moles of atoms of the same elements. For example, acetylene, C2H2, and benzene, CeHe, each have 1 1 ratios of moles of carbon atoms to moles of hydrogen atoms. That is, each has an empirical formula CH. Such compounds have the same percent compositions. However, they do not have the same number of atoms in each molecule. The molecular formula is a formula that gives all the information that the empirical formula gives (the mole ratios of the various elements) plus the information of how many atoms are in each molecule. In order to deduce molecular formulas from experimental data, the percent composition and the molar mass are usually determined. The molar mass may be determined experimentally in several ways, one of which will be described in Chap. 12. 

This is an extension of an ideal gas law calcnlation involving molar mass. If you determine the molar mass of the gas, yon will be able to determine the molecular formula from the empirical formula. 

J 3 Determine the molecular formula of a compound from its empirical formula and its molar mass (Example F.3). 

F.13 Osmium forms a number of molecular compounds with carbon monoxide. One light-vellow compound was analyzed to give the following elemental composition 15.89% C, 21.18% O, and 62.93% Os. (a) What is the empirical formula of this compound (b) From the mass spectrum of the compound, the molecule was determined to have a molar mass of 907 g-mol 1. What is its molecular formula ... 

The flowchart in Figure 3-15 outlines the process. From masses of products, determine masses of elements. Then convert masses of elements to moles of elements. From moles of the elements, find the empirical formula. Finally, use information about the molar mass to obtain the molecular formula. 

Take the freezing-point constant from Table 8.9. Use this -molality to calculate the moles of solute in the sample by multiplying it by the mass of solvent in kilograms. At this stage, determine the molar mass of the solute by dividing the given mass of solute by the number of moles present. For the molecular formula, decide how many atoms of sulfur are needed in each molecule to account for the molar mass. 

If we calculated the percent compositions of C2H2 and CeHg (Figure 7.3), we would find that both have the same percentages of carbon and the same percentages of hydrogen (compare Problem 7.100 at the end of the chapter). Both have the same empirical formula—CH. This result means that we cannot tell these two compounds apart from percent composition data alone. However, if we also have a molar mass, we can use that information with the percent composition data to determine not only the empirical formula but also the molecular formula. 

The molar mass of a gas may be calculated if the mass of a sample and the number of moles of the sample are both known. The ideal gas law may be used to determine the number of moles, from which the molar mass may be calculated. As introduced in Section 7.4, the molar mass, along with the empirical formula, may then be used to determine the molecular formula (Section 12.7). 

The molecular formula is determined from the empirical formula and the experimentally determined molar mass. 

The molecular formula for a compound can be determined by finding the integer by which the mass of the empirical formula differs from the molar mass of the compound. 

The molecular formula is some whole-number multiple of the empirical formula. To determine the molecular formula, you must know the approximate molar mass of the compound under study. From Avogadro s hypothesis, the ratio of molar masses of two gaseous compounds is the same as the ratio of their densities, provided that those densities are measured at the same temperature and pressure. (This is true because a given volume contains the same number of molecules of the two gases.) The density of the welding gas from Example 2.4 is 1.06 g at 25°C... 

The simplest formula may also be the molecular formula, but not necessarily. The true molecular formula may be determined from the molar mass and the simplest formula. Consider Example C.5. 

Plan We find the masses of CO2 and H2O by subtracting the masses of the absorbers before the reaction from the masses after. From the mass of CO2, we use the mass fraction of C in CO2 to find the mass of C (see Comment in Sample Problem 3.3). Similarly, we find the mass of H from the mass of H2O. The mass of vitamin C (I.OOO g) minus the sum of the C and H masses gives the mass of O, the third element present. Then, we proceed as in Sample Problem 3.5 calculate numbers of moles using the elements molar masses, construct the empirical formula, determine the whole-number multiple from the given molar mass, and construct the molecular formula. 

From the masses of elements in an unknown compound, the relative amounts (in moles) can be found and the empirical formula determined. If the molar mass is known, the molecular formula can also be determined. Methods such as combustion analysis provide data on the masses of elements in a compound, which can be used to obtain the formula. Because atoms can bond in different arrangements, more than one compound may have the same molecular formula (constitutional isomers). 

Determining Molecular Formula from Mass Percent and Molar Mass... 

The calculation of the molar mass from the molecular formula gives the same result as the given, experimentally-determined molar mass. 

The empirical formula can be calculated from mass percent data. The molar mass can be calculated using the ideal gas equation. The molecular formula can then be determined. 

Strategy First, we can determine the empirical formula from mass percent data. Then, we can determine the molar mass from the freezing-point depressioiL Finally, from the empirical formula and the molar mass, we can find the molecular formula. 

In the next section, we will explore in more detail how to calculate the empirical formula for a compound from the relative masses of the elements present. As we will see in Sections 8.8 and 8.9, we must know the molar mass of a compound to determine its molecular formula. 

Check Note that in determining the molecular formula from the empirical formula, we need only know the approximate molar mass of the componnd. The reason is that the true molar mass is an integral multiple (IX, 2X, 3X,. . . ) of the empirical molar mass. Therefore, the ratio (molar mass/empirical molar mass) will always be close to an integer. 

Strategy This problem can be divided into two parts. First, it asks for the empirical formula of the compound from the percent by mass of Si and F. Second, the information provided enables us to calculate the molar mass of the compound and hence determine its molecular formula. What is the relationship between empirical molar mass and molar mass calculated from the molecular formula ... 

Strategy Solving this problem requires three steps. First, we calculate the molality of the solution from the depression in freezing point. Next, from the molality we determine the number of moles in 7.85 g of the compound and hence its molar mass. Finally, comparing the experimental molar mass with the empirical molar mass enables us to write the molecular formula. 

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 ... 

---