Determining the Formula of an Unknown Compound

From here on, you should be able to determine the molar mass of a compound, so that calculation will no longer be shown. 

FOLLOW-UP PROBLEM 3.3 Ammonium nitrate is a common fertilizer. Agronomists base the effectiveness of fertilizers on their nitrogen content. 

A mole of substance is the amount that contains Avogadro s number (6.022x10 ) of chemical entities (atoms, molecules, or formula units). The mass (in grams) of a mole has the same numerical value as the mass (in amu) of the entity. Thus, the mole allows us to count entitles by weighing them. Using the molar mass (jM., g/mol) of an element (or compound) and Avogadro s number as conversion factors, we can convert among amount (mol), mass (g), and number of entities. The mass fraction of element X in a compound is used to find the mass of X in any amount of the compound. 

In Sample Problem 3.3, we knew the formula and used it to find the mass percent (or mass fraction) of an element in a compound and the mass of the element in a given mass of the compound. In this section, we do the reverse use the masses of elements in a compound to find its formula. We ll present the mass data in several ways and then look briefly at molecular structures. 

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Substances that have the same crystal structure have similar chemical formulae. The law can be used to determine the formula of an unknown compound if it is isomorphous with a compound of known formula. It is named after Eilhard MitscherRch (1794-1863). 

Defining the Mole 72 Determining Molar Mass 73 Converting Between Amount, Mass, and Number of Chemical Entities 74 The Importance of Mass Percent 77 Determining the Formula of an Unknown Compound 80 Empirical Formulas 80 Molecular Formulas 81 Isomers 84... 

Determining the identity of an unknown compound is important in all kinds of research. It can even be used to solve crimes. Forensic scientists specialize in analyzing evidence for criminal and legal cases. To understand why forensic scientists might need to find out the molecular formula of a compound, consider the following example. 

Example Using its NMR spectrum, determine the structure of an unknown compound X that has moiecular formula C4H8O2 and contains a C=0 absorption in its IR spectrum. 

Sometimes a chemical reaction gives a product that has never been obtained before. In such a case, a chemist determines what compound has been formed by determining which elements are present and how much of each is there. These data can be used to obtain the formula of the compound. Previously we used the formula of the compound to determine the mass of each element present in a mole of the compound. To obtain the formula of an unknown compound, we do the opposite. That is, we use the measured masses of the elements present to determine the formula. 

Start at 4000 cm and move down the wavenumber scale. Make note of characteristic peaks, especially those that are unique to certain functional groups. Observe that the absence of peaks also provides clues for the types of functional groups that cannot be present. Once all the possible functional groups have been identified, propose chemical structures using these functional groups and the elements provided by the molecular formula. In Section 11.4G, we learn the concept of index of hydrogen deficiency, which can also be used in these types of problems to determine the structure of an unknown compound. 

Determine possible structures for the same spectrum (above) for a compound with molecular formula CsHiqO. What does example 11.7 and this problem tell you about the effectiveness of IR spectroscopy for determining the structure of an unknown compound ... 

Mass spectroscopy is often the first tool in determining the identity of an unknown compound because simple (low resolution) mass spectroscopy usually yields an accurate molecular formula. 

O QZ9 Explain the role that a mass spectrometer plays in determining the molecular formula of an unknown compound. 

The empirical formula of an unknown compound must be determined by experiment. 

What information must a chemist obtain in order to determine the empirical formula of an unknown compound (11.4)... 

Because the percent composition by mass can be obtained experimentally by a number of methods, the procedure outlined in Example 0.16 enables us to determine the empirical formula of an unknown compound. In fact, the word empirical literally means based only on observation and measurement. ... 

Applying these steps mm/ allow assignment of the structure of an unknown compound. However, without additional information about a substance, such as its molecular formula or the chemical reaction(s) by which it was formed, only a partial structural determination may be possible. Other chemical or spectral data may be required to make a complete assignment. In any case, structural assignments based exclusively on interpretation of spectra must be confirmed by comparing the spectra of the unknown with those of an authentic specimen, if the compound is known, or by chemically converting the unknown to a known compound. 

High-resolution mass spectrometry involves the use of a detector that can measure tw/z values to four decimal places. This technique allows for the determination of the molecular formula of an unknown compound. In order to analyze the data obtained from high-resolution mass spectrometry, we must first review some background information. Specifically, we must discuss why atomic masses are not whole numbers (despite the fact that we have treated them as such until now). 

When measured to four decimal places, the mass of these compounds are in fact diflFerent, and this difference is detectable. In a high-resolution mass spectrometer, the molecular ion for the first compound will appear near mlz= 84.0573, while the molecular ion of the second compound will appear near mlz= 84.0936. In this way, the molecular formula of an unknown compound can be determined via high-resolution mass spectrometry. The mass of the molecular ion is measured accurately, and a simple computer program can then calculate the correct molecular formula. The computer program is not entirely necessary, because published data tables can be cross-referenced to find the matching molecular formula. 

In this section, we will use EI-MS data to determine the structure of two unknown compounds. Even if the compound you are analyzing is not in searchable MS databases available to you, it is still possible to determine the structure of the compound with a few key pieces of data. If the molecular formula is available, either from an exact mass determination (Section 3.6) or Rule of Thirteen analysis (Section 1.5) on the molecular ion, the process is much simpler. Furthermore, knowing the main functional group(s) in the compound will assist in analyzing the fragmentation pattern. Information from an infrared spectrum and/or NMR spectra are useful in this regard. 

How do you approach determining a structure of an unknown compound utilizing carbon and proton NMR spectra Let s look at the proton NMR spectrum shown in Figure 6.25. The spectrum is for a compound with formula C6H10O2. The index of hydrogen deficiency for this compound is calculated to be 2. 

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