Formulas of a compound

Wnte the structural formula of a compound of molecular formula C4HgCl2 in which... 

Beginning with the fourth alkane, butane, we find we can draw a structural formula of a compound with four atoms and ten hydrogen atoms in two ways the first is as the normal butane exists and the second is as follows, with the name isobutane (refer to Table 1 for properties). 

As you will see shortly, the formula of a compound can be used to determine the mass percents of the elements present. Conversely, if the percentages of the elements are known, the simplest formula can be determined. Knowing the molar mass of a molecular compound, it is possible to go one step further and find the molecular formula. In this section we will consider how these three types of calculations are carried out. 

Knowing the formula of a compound, Fe203> you can readily calculate the mass percents of its constituent elements. It is convenient to start with one mole of compound (Example 3.4a). The formula of a compound can also be used in a straightforward way to find the mass of an element in a known mass of the compound (Example 3.4b). 

The most complex problem of this type requires you to determine the simplest formula of a compound given only the raw data obtained from its analysis. Here, an additional step is involved you have to determine the masses of the elements present in a fixed mass of the compound (Example 3.6). 

Chemical analysis always leads to the simplest formula of a compound because it gives only the simplest atom ratio of the elements. As pointed out earlier, the molecular formula is a whole-number multiple of the simplest formula. That multiple may be 1 as in H20, 2 as in H202, 3 as in CjHg, or some other integer. To find the multiple, one more piece of data is... 

Use the formula of a compound to find percent composition or its equivalent (Example 3.4 Problems 33-42) 34,36... 

Find the simplest formula of a compound from chemical analysis. 

Write the formula of a compound of each of the following elements that cannot act as an oxidizing agent. 

Simplest formula A formula of a compound which gives the simplest atom ratio of the elements present The simplest formula of hydrogen peroxide is HO, 57-60... 

The chemical formula of a compound represents its composition in terms of chemical symbols. Subscripts show the numbers of atoms of each element present in the smallest unit that is representative of the compound. For molecular compounds, it is common to give the molecular formula, a chemical formula that shows how many atoms of each type of element are present in a single molecule of the compound. For instance, the molecular formula for water is H20 each molecule contains one O atom and two H atoms. The molecular formula for estrone, a female sex hormone, is Clgl-I2202, showing that a single molecule of estrone consists of 18 C atoms, 22 FI atoms, and 2 O atoms. A molecule of a male sex hormone, testosterone, differs by only a few atoms its molecular formula is (lyH2802. Think of the consequences of that tiny difference ... 

To determine the empirical formula of a compound, we begin by measuring the mass of each element present in a sample. The result is usually reported as the mass... 

If the chemical formula of a compound is already known, its mass percentage composition can be obtained from the formula. 

The empirical formula of a compound is determined from the mass percentage composition and the molar masses of the elements present. 

To find the molecular formula of a compound, we need one more piece of information—its molar mass. Then all we have to do is to calculate how many empirical formula units are needed to account for the molar mass. One of the best ways of determining the molar mass of an organic compound is by mass spectrometry. We saw this technique applied to atoms in Section B. It can be applied to molecules, too and, although there are important changes of detail, the technique is essentially the same. 

The molecular formula of a compound is found by determining how many empirical formula units are needed to account for the measured molar mass of the compound. 

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

What is the molecular formula of a compound of empirical formula CH that diffuses 1.24 times more slowly than krypton at the same temperature and pressure ... 

The chemical formula of a compound contains essential information about its composition. The formula identifies which elements are present, and it states the number of atoms of each kind present in one unit of the compound. We need the chemical formula of a substance to calculate its molar mass. In fact, almost all chemical calculations require the correct chemical formula. How are chemical formulas determined in the first place ... 

This example shows how to compute mass percentages from a chemical formula. When the formula of a compound is unknown, chemists must work in the opposite direction. First, they do experiments to find the mass percentage of each element, and then they deduce what chemical formula matches those percentages. 

This flowchart summarizes the method for using combustion analysis to determine the empirical formula of a compound that contains no elements other than C, H, and O. 

Example 3-15 demonstrates how to use combustion analysis to determine the formula of a compound containing only C and H, and Example shows how combustion analysis is conducted when the compound contains O. 

The concept of percentage is often used to describe the composition of compounds. If the formula of a compound is known, the percent by mass of an element in the compound is determined by computing the fraction of the formula weight which is made up of that element, and multiplying that fraction by 100%. Thus, an element X with atomic weight 40.0 amu in a compound XY of formula weight 99.0 amu will be present in... 

EXAMPLE 4.12. What is the empirical formula of a compound which contains 40.0% sulfur and 60.0% oxygen, by mass ... 

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

The empirical formula is the simplest formula of a compound. It shows the ratio of the elements present in smallest whole numbers. 

To find the molecular formula of a compound, both the empirical formula and the molar mass (in g mol-1) of the compound must be known. 

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