Determining the Molecular Formula

Butane and acetone both have a molecular mass of 58. Calculate the exact masses of these compounds and explain whether they can be distinguished by high-resolution mass spectrometry. 

These values are not the same as the atomic masses in the periodic table because these are the exact masses of individual isotopes.The masses in the periodic table are average masses of the element based on the masses and natural abundances of the isotopes of which it is composed. 

CHAPTER 15 ULTRAVIOLET-VISIBLE SPECTROSCOPY AND MASS SPECTROMETRY 

Most elements are composed of more than one isotope. In the case of carbon the relative abundances of the two major isotopes, l2C and l3C, are 100 to 1.1. In a sample of methane, then, for every 100 molecules composed of l2CH4 there are 1.1 molecules of l3CH4. Therefore, in the mass spectrum of methane the mlz 17 peak, which is due to 13CH4, has 1.1% of the intensity of the mlz 16 peak, which is due to 12CH4. Benzene has six carbons, each with a 1.1% chance of being l3C. Therefore, the M + 1 peak at mlz 79 should be approximately 6 X 1.1 = 6.6% of the M+ peak at 78 mlz, in good agreement with the observed value of 6.8%. The M + 2 peak, with 0.2% of the intensity of the Mt peak, is due primarily to molecules that contain two atoms of 13C. 

What are the predicted intensities of the M + 1 peaks in the mass spectra of butane and acetone, relative to the intensity of the peak Do you think that these intensities could be used to distinguish between these compounds  

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The molar mass of acetic acid, as determined with a mass spectrometer, is about 60 g/mol. Using that information along with the simplest formula found in Example 3.6, determine the molecular formula of acetic acid. 

EXAMPLE F.3 Sample exercise Determining 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). 

C03-0005. Determine the molecular formula, stmctural formula, and line stmcture for each compound whose ball-and-stick model follows (see Figure for atom colors) ... 

C03-0073. Determine the molecular formula and calculate the molar mass of each of the following essential amino acids ... 

C03-0126. A 6.82-g sample of a compound that contains silicon and chlorine is 16.53% by mass silicon. The molar mass Is less than 200 g/mol. Determine the molecular formula of the compound. 

Determine the molecular formulas, name these compounds, and draw the structures of the platinum complexes. 

Determine the molecular formula of a compound with molecular weight between 50 and 60 which contains 88.8% C and 11.2% H. 

We determine the molality of the solution, then the number of moles of solute present, and then the molar mass of the solute. Then we determine the compound s empirical formula, and combine this with the molar mass to determine the molecular formula. 

Problem 23 The ozone molecule contains only oxygen atoms. Determine the molecular formula of ozone given that 2.3 grams occupies 1,073 milliliters at standard temperature and pressure. 

If these isotope peaks are intense enough to be measured accurately, the above calculations may be useful in determining the molecular formula. ... 

For most of the Problems in this text, the unit-resolution molecular ion, used in conjunction with IR and NMR, will suffice for determining the molecular formula by browsing in Appendix A. For several more difficult Problems, the high-resolution formula masses— for use with Appendix A (see Section 2.4.2)—have been supplied. 

Mass analysis is the process by which the mass defect" can be used to determine the molecular formula of an ion. 

Determine the molecular formula of a compound of molecular weight 80 and elemental percentage composition by weight of C = 45.00, H = 7.50, and F = 47.45. Write structures for all the possible isomers having this formula. (See Exercise 1-15 for a description of how percentage composition is determined by combustion experiments.)... 

With the molecular weight available from the M+ peak with reasonable certainty, the next step is to determine the molecular formula. If the resolution of the instrument is sufficiently high, quite exact masses can be measured, which means that ions with mje values differing by one part in 50,000 can be distinguished. At this resolution it is possible to determine the elemental composition of each ion from its exact m/e value (see Exercise 9-44). 

A formula such as C4H9, which is determined from data about percent composition, is called an empirical formula because it tells only the ratios of atoms in a compound. The molecular formula, which tells the actual numbers of atoms in a molecule, can be either the same as the empirical formula or a multiple of it, such as C8H18. To determine the molecular formula, it s necessary to know the molecular mass of the substance. In the present instance, the molecular mass of our compound (a substance called octane) is 114.2 amu, which is a simple multiple of the empirical molecular mass for C4H9 (57.1 amu). 

Elemental analysis can provide only an empirical formula. To determine the molecular formula, it is also necessary to know the substance s molecular mass. In the present problem, the molecular mass of naphthalene is 128.2 amu, or twice the empirical formula mass of C5H4 (64.1 amu). Thus, the molecular formula of naphthalene is C(2x5)H(2x4)/ or Ci0H8. 

The chemical makeup of a substance is described by its percent composition—the percentage of the substance s mass due to each of its constituent elements. Elemental analysis is used to calculate a substance s empirical formula, which gives the smallest whole-number ratio of atoms of the elements in the compound. To determine the molecular formula, which may be a simple multiple of the empirical formula, it s also necessary to know the substance s molecular mass. Molecular masses are usually determined by mass spectrometry. 

One limitation on the use of isotope peak intensities to determine the molecular formula is that the molecular ion must be relatively intense, otherwise the isotope peaks will be too weak to be measured with the necessary accuracy. Difficulty may also arise from spurious contributions to the isotope peak intensities from the protonated molecular ion, from weak background peaks or from impurities in the sample. In any event the method is only reliable for molecules having molecular weights up to about 250-300. 

In many areas of science (chemistry, biochemistry, etc.), and particularly in drug metabolism studies, there is a need to determine the molecular formula of metabolites of interest. This is where the use of MS for accurate mass measurement has transformed the way in which scientists work. Today, it is very easy to obtain an exact mass measurement of a known or unknown substance. 

In 1834, Eilhardt Mitscherlich conducted vapor density measurements on benzene. Based on data from these experiments, he determined the molecular formula of benzene to be C6H6. This formula suggested that the benzene molecule should possess four modes of unsaturation because the saturated alkane with six carbon atoms would have a formula of C6HI4. These unsaturations could exist as double bonds, a ring formation, or a combination of both. 

Note that for unknown impurities, high resolution LC—MS may be used to determine the molecular formula (25-27). For molar (rather than weight) RRF values, one needs only the relative number of nitrogens per molecule and not the molecular weight. 

The procedure of dividing n(E) by n(C) is equivalent to finding the number of atoms of each element for each atom of carbon. The ratio of H to C atoms is 2 1. This means that the empirical formula is CH2 and that the molecular formula will be a multiple of CH2 that is, if we had the information to determine the molecular formula, which we don t. 

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 ... 

For each exact mass corresponding to mass spectra A-W, determine the molecular formula. Remember to look at the heteroatoms that were determined in exercise 1.7. 

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