Ionic substances empirical formulas

Molecular formulas give all the information that empirical formulas do, plus the ratio of the number of moles of each element to the number of moles of the compound. (Molecular formulas are used only for molecular substances, not ionic substances.) A molecular formula can be determined from the empirical formula of the compound and its formula mass First, divide the formula mass by the mass in amu of one empirical formula unit, which will result in a small integer. Then, multiply each subscript of the empirical formula by that integer. (Section 7.5)... 

An empirical formula shows the simplest ratio of atoms in a molecule (or an ionic compound). For a molecular substance, the empirical formula only tells you the ratio of atoms, so it may not be very helpful at identifying the nature of a substance. For example, take any of the group of hydrocarbons known as the alkenes. All of the different compounds have a l-to-2 ratio of carbon to hydrogen. Ethene is C2H4, propene is C H, and butene is C,Hj. If you reduce all of the subscripts, they each come out to CH2. Knowing the empirical formula helps to tell you that this substance is an alkene, but it doesn t tell you which one. How can you determine what a compound is from the empirical formula The molecular weight will solve your problem. If you know the empirical formula and the molecular weight, you can determine the molecular formula for a compound. The first type of calculation we will look at is the determination of the empirical formula. The second type is the determination of a molecular formula. 

An ionic compound is an aggregrate of ions, these being present in a definite ionic ratio that is characteristic of the compound. Since there are no discrete molecules in the infinite lattice of an ionic substance, only an empirical formula can be used to express its composition. Thus, for nickel(II) chloride, which is composed of a collection of Ni t and Cl " ions in the ratio of one to two, the empirical formula is NiCl2. There is no molecular formula for nickel(II) chloride. 

The ions in ionic compounds are arranged in three-dimensional structures, as Figure 2.21(b) shows for NaCl. Because there is no discrete molecule of NaCl, we are able to write only an empirical formula for this substance. This is true for most other ionic compounds. 

The percentage composition of a compound leads directly to its empirical formula. An empirical formula (or simplest formula) for a compound is the formula of a substance written with the smallest integer (whole number) subscripts. For most ionic substances, the empirical formula is the formula of the compound. This is often not the case for molecular substances. For example, hydrogen peroxide has the molecular formula H2O2. The molecular formula, you may recall, tells you Ihe precise number of atoms of different elements in a molecule of the substance. The empirical formula, however, merely tells you the ratio of numbers of atoms in the compound. The empirical formula of hydrogen peroxide (H2O2) is HO (Figure 3.7). 

It is known as the empirical formula of the substance and can be applied to ionic and covalent compounds. 

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. 

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