Definition of empirical formula
A notation indicating the ratios of the various elements present in a compound, without regard to the actual numbers.
Examples of empirical formula in the following topics:
- Although a chemical formula may imply certain simple chemical structures, it is not the same as a full chemical structural formula.
- Molecular formulas indicate the simple numbers of each type of atom in a molecule of a molecular substance, and are thus sometimes the same as empirical formulas (for molecules that only have one atom of a particular type), and at other times require larger numbers than do empirical formulas.
- An example of the difference is the empirical formula for glucose, which is CH2O, while its molecular formula requires all numbers to be increased by a factor of six, giving C6H12O6.However, as can be seen by comparing the molecular formula to the structural formula , the former lacks information about the arrangement of atoms; because of this, one molecular formula can describe a number of different chemical structures.
- The molecular formula for a compound can be the same as or a multiple of the compound's empirical formula, as the empirical formula represents the simplest whole-integer ratio of atoms in a compound.
- Combustion analysis can be used to determine the empirical formula of a compound, and if the molecular weight of the compound is known, this information can also be used to determine the molecular formula.Examples of Molecular Formulas:The compound dichlorine hexoxide has an empirical formula ClO3, and molecular formula Cl2O6The compound glucose has the empirical formula CH2O, and the molecular formula C6H12O6
- Empirical formulas are the simplest form of notation.
- The molecular formula for a compound is either equal to or a whole-number multiple of its empirical formula.
- As does a molecular formula, an empirical formula lacks any structural information about the positioning or bonding of atoms in a molecule and can therefore describe a number of different structures, or isomers, with varying physical properties.
- For butane and isobutane (shown below), the empirical formula for both molecules is C2H5, and they share the same molecular formula of C4H10.
- This formula also happens to be methyl acetate's molecular formula.
- Empirical formulas describe the simplest whole-number ratio of the elements in a compound.
- The atomic composition of chemical compounds can be described using a variety of notations including molecular, empirical, and structural formulas.
- Another convenient way to describe atomic composition is to examine the percent composition of a compound by mass.Percent composition is calculated from a molecular formula by dividing the mass of a single element in one mole of a compound by the mass of one mole of entire compound.
- For example, butane has an empirical formula of C2H5 and a molecular formula of C4H10 .Butane's percent composition can be calculated as follows:Mass of H per mol butane: 10 mol H x 1.0079 g H/mol H = 10.079 g HMass of C per mol butane: 4 mol C x 12.011 g C/mol C = 48.044 g CMass percent H in butane: 10.079 g H / 58.123 g butane x 100 = 17.3% HMass percent C in butane: 48.044 g C / 58.123 g butane x 100 = 82.7% CTherefore, the atomic composition of butane can also be described as 17.3% hydrogen and 82.7% carbon, and as expected, these values sum to 100%.In practice, this calculation is often reversed.
- Mass percents can be determined experimentally via elemental analysis, and these values can be used to calculate the empirical formula of unknown compounds.
- However, this information is insufficient to determine the molecular formula without additional constraints on the compound's molecular weight.
- Often, a compound's composition can also be denoted by an empirical formula, which is the simplest integer ratio of its constituent chemical elements, but this empirical formula does not always describe the specific molecule in question since it provides only the ratio of its constituent elements.
- The elemental composition of a molecule can be exactly represented by its molecular formula, which provides the exact number of atoms that are in the molecule.
- A compound's empirical formula is the simplest integer ratio of the chemical elements that constitute it.
- For example, carbohydrates have the same ratio (carbon: hydrogen: oxygen = 1:2:1) (and thus the same empirical formula) but different total numbers of atoms in the molecule.
- Molecular and Empirical FormulasThe molecular formula reflects the exact number of atoms that compose the molecule and so characterizes different molecules.
- The empirical formula is often the same as the molecular formula but not always.
- For network solids, the term formula unit is used in stoichiometric calculations.
- In addition, though any ratio of 2 bromines to 1 magnesium atom will satisfy the two requirements above, the formula for ionic compounds in typically presented as the empirical formula, or the simplest ratio of atoms involving positive integers.
- Combustion analysis is commonly used to analyze samples of unknown chemical formula and requires only milligrams of a sample.
- These data can then be used to calculate the empirical formula of the unknown sample.
- Trivalent aluminum compounds are therefore exceptionally useful as Lewis acids, particularly in organic synthesis.Aluminum Hydrides and Organoaluminum Compounds A variety of compounds of empirical formula AlR3 and AlR1.5Cl1.5 exist.