Percentage composition determining molecular formula from

Sometimes there are more than two elements in a compound, or more than one atom of each element. This makes determining percentage composition more complex than in the example above. Work through the Sample Problem below to learn how to calculate the percentage composition of a compound from its molecular formula. 

Because any two compounds with the same empirical formula will give the same percent composition, it is impossible to uniquely determine the molecular formula from the percent compositions by mass alone. For example, both acetylene (C2H2) and benzene (CgHg) have the same empirical formula (CH), so any experiment that only gives the percent composition would be unable to distinguish between the two compounds. Therefore, we need to independently determine both the percentage... 

When a chemist has discovered a new compound, the first question to answer is. What is the formula To answer, you begin by analyzing the compound to determine amounts of the elements for a given amount of compound. This is conveniently expressed as percentage composition—that is, as the mass percentages of each element in the compound. You then determine the formula from this percentage composition. If the compound is a molecular substance, you must also find the molecular weight of the compound in order to determine the molecular formula. 

Determining the Molecular Formula from Percentage Composition and Molecular Weight... 

Determining the molecular formula from percentage composition and molecular weight Given the empirical formula and molecular weight of a substance, obtain its molecular formula. (EXAMPLE 3.12)... 

Table 1.2 shows how to determine the empirical formula of a compound from the percentage compositions determined in an analysis. Remember that the empirical formula expresses the simplest whole-number ratios of the elements and may need to be multiplied by an integer to obtain the true molecular formula. To determine the value of the multiplier, a molecular mass is required. Determination of the molecular mass is discussed in the next section. 

In the problem above, we determined the percentage data from the chemical formula. We can determine the empirical formula if we know the percent compositions of the various elements. The empirical formula tells us what elements are present in the compound and the simplest whole-number ratio of elements. The data may be in terms of percentage, or mass or even moles. However, the procedure is still the same—convert each element to moles, divide each by the smallest, and then use an appropriate multiplier if necessary. We can then determine the empirical formula mass. If we know the actual molecular mass, dividing the molecular formula mass by the empirical formula mass, gives an integer (rounded if needed) that we can multiply each of the subscripts in the empirical formula. This gives the molecular (actual) formula, which tells what elements are in the compound and the actual number of each. 

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. 

This only expresses the relative number of atoms of each element in a compound. Nevertheless, it is the first step in the experimental determination of the molecular formula of a compound from its percentage composition. 

To obtain the molecular formula of a substance, you need two pieces of information (1) as in the previous section, the percentage composition, from which the empirical formula can be determined and (2) the molecular weight The molecular weight allows you to choose the correct multiple of the empirical formula for the molecular formula. We will illustrate these steps in the next three examples. 

When a new substance is synthesized or is discovered, it is analyzed quantitatively to reveal its percentage composition. From these data, the empirical formula is then determined. An empirical formula consists of the symbols for the elements combined in a compound, with subscripts showing the smallest whole-number mole ratio of the different atoms in the compound. For an ionic compound, the formula unit is usually the compound s empirical formula. For a molecular compound, however, the empirical formula does not necessarily indicate the actual numbers of atoms present in each molecule. For example, the empirical formula of the gas diborane is BHg, but the molecular formula is BgHg. In this case, the number of atoms given by the molecular formula corresponds to the empirical ratio multiplied by two. 

A.4 (a) To calculate the percentage composition from the molecular formula, first determine the weight of each element in 1 mol of the compound. For C6H12O6,... 

Given only a molecular formula (perhaps derived from an experimentally determined percentage composition and a molecular weight determination) and some chemical evidence that suggests what functional groups might be present, how might we proceed to propose a possible structure ... 

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