structural formula: see formula.

molecular formula: see formula.

formula weight, in chemistry, a quantity computed by multiplying the atomic weight (in atomic mass units) of each element in a formula by the number of atoms of that element present in the formula, and then adding all of these products together. For example, the formula weight of water (H₂O) is two times the atomic weight of hydrogen plus one times the atomic weight of oxygen. Numerically, this is (2×1.00797)+(1×15.9994)=2.01594+15.9994=18.01534. If the formula used in computing the formula weight is the molecular formula, the formula weight computed is the molecular weight. The percentage by weight of any atom or group of atoms in a compound can be computed by dividing the total weight of the atom (or group of atoms) in the formula by the formula weight and multiplying by 100. For example, the weight percentage of hydrogen in water is determined by taking two times the atomic weight of hydrogen, dividing it by the formula weight of water, and multiplying by 100. Numerically, this is 100×(2×1.00797)/18.01534=11.19% hydrogen in water by weight. Formula weights are especially useful in determining the relative weights of reagents and products in a chemical reaction. For example, it is known that two molecules of hydrogen gas, H₂, react with one molecule of oxygen gas, O₂, to form two molecules of water, H₂O. This reaction may be represented by the chemical equation 2H₂+O₂→2H₂O. The formula weight of hydrogen gas is 2.01594, that of oxygen gas 31.9998, and that of water 18.01534. Our chemical equation is numerically equivalent to 2×2.01594+31.9998=2×18.01534 or 4.03188+31.9998=36.03068 if the formula weight of each reactant is substituted for the formula of that reactant. From this equation we know, for example, that 4.03188 grams of hydrogen gas will react with 31.9998 grams of oxygen gas to yield 36.03068 grams of water. The relative proportions by weight of these reactants is the same in any reaction of hydrogen and oxygen to form water. These relative weights computed from the chemical equation are sometimes called equation weights.

formula, in chemistry, an expression showing the chemical composition of a compound. Formulas of compounds are used in writing the equations (see chemical equations) that represent chemical reactions. Compounds are combinations in fixed proportions of the chemical elements. The smallest unit of an element is the atom.

Formulas for Compounds

The formula of a well-known compound, water, is H₂O. Water is made up of molecules, and the formula shows that each molecule consists of two atoms of hydrogen, H, bonded to an atom of oxygen, O. The subscript 2 indicates that there are two atoms of hydrogen in the molecule; where no subscript appears, as after the O, the subscript 1 is implied. It should be kept in mind that not all compounds are molecular. For example, sodium chloride, NaCl, is an ionic rather than a molecular compound. Solid sodium chloride consists of a collection of sodium ions and chloride ions arranged in a regular, three-dimensional pattern called a crystalline structure. One cannot say that a certain sodium ion and a certain chloride ion are grouped together into a unit, since each sodium ion is equally associated with all its neighboring chloride ions and each chloride ion is equally associated with all its neighboring sodium ions. The formula NaCl, therefore, cannot be taken as showing the composition of some particular unit, such as a molecule. Rather, it shows the proportion of the atoms of each element making up the compound—in this case, one atom of sodium to every atom of chlorine; such a formula is called an empirical formula.

Molecular and Empirical Formulas

If a compound is molecular, the molecular formula is preferred to the empirical formula since it gives more information. A molecule of glucose, for example, consists of 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Its molecular formula, C₆H₁₂O₆, displays this information explicitly; the empirical formula is CH₂O. From the formula one can also deduce the proportion of the atoms of each element making up the compound: one atom of carbon to every two atoms of hydrogen to every one atom of oxygen (6 : 12 : 6=1 : 2 : 1). The empirical formula of glucose, CH₂O, shows only the proportion, not the actual number of atoms.

Many compounds may have the same empirical formula. For example, formaldehyde, each molecule of which consists of one carbon atom, two hydrogen atoms, and one oxygen atom, has the molecular formula CH₂O, which is identical to the empirical formula of glucose. Another example is furnished by ethyne (acetylene), whose molecular formula is C₂H₂, and benzene, whose molecular formula is C₆H₆. Both have the same empirical formula, CH.

In addition to showing the actual number of atoms, molecular formulas are also more useful than empirical formulas in that they explicitly show radicals. For example, the molecular formula for the compound aluminum sulfate, Al₂(SO₄)₃, shows that it contains three sulfate radicals (SO₄). The empirical formula, Al₂S₃O₁₂, does not show this. When only one radical is present in the molecule, the parentheses and subscript are omitted, e.g., CuSO₄ for cupric sulfate. Other groups are also shown in molecular formulas, e.g., the water molecules in the mineral chalcanthite (blue vitriol), which consists of cupric sulfate atoms to each of which are attached five water molecules. Its molecular formula is CuSO₄·5H₂O, its empirical formula CuSO₉H₁₀.

Structural Formulas

In many cases, especially with organic compounds, even the molecular formula does not provide enough information to identify a compound, so that structural formulas are needed. For example, both ethanol (ethyl alcohol) and dimethyl ether have the molecular formula C₂H₆O (see isomer). Their structural formulas are:In these formulas each line represents a single covalent chemical bond. A double bond is represented by a double line and a triple bond by a triple line. In ethene (ethylene), C₂H₄, the carbon atoms are joined by a double bond. The structural formula of ethene is:(In many representations of structural formulas, the angles of the lines indicating bonds do not necessarily have meaning.) In ethyne (acetylene), C₂H₂, the carbon atoms are joined by a triple bond. The structural formula of ethyne is:

Semistructural Formulas

Structural formulas are often simplified so that they can be written on a single line; the simplified formulas are often called semistructural formulas. The semistructural formula for ethanol is CH₃CH₂OH, or more simply C₂H₅OH. In such a semistructural formula the OH is written explicitly to indicate that the oxygen has a hydrogen bonded to it. The C₂H₅ indicates that the two carbon atoms are bonded to one another. The semistructural formula for dimethyl ether may be written CH₃OCH₃. Here the O is placed between the two carbon atoms to show that the carbons are bonded to the oxygen. A carbon often has three hydrogens bonded to it, and the H₃ is written after the C. In some cases the H₃ is written before the C for clarity; thus the formula for dimethyl ether might be written H₃COCH₃.

Electron Dot Diagrams

Dots are used in a type of formula called the electron dot diagram, where each pair of dots represents a pair of shared electrons in a covalent bond. The diagrams for ethane (CH₃CH₃), ethene, and ethyne are:

formula, in mathematics and physics, equation expressing a definite fixed relationship between certain quantities. The quantities are usually expressed by letters, and their relationship is indicated by algebraic symbols. For example, A=πr² is the formula for the area A of a circle of radius r, and s=1/2at² is the formula for the distance s traveled by a body experiencing an acceleration a during a time interval t.

empirical formula: see formula.

Formula One (F1), type of racecar used in Grand Prix automobile racing. Capable of speeds exceeding 230 mph (370 kph), the technologically sophisticated F1 cars are low-slung, open-wheeled, single-seat vehicles with powerful mid-engines, air foils, electronic aids, special suspensions, and large tires. They are usually smaller and more maneuverable than similar "Indy-type" racecars. Grand Prix races are usually held on special closed-circuit racetracks, although some (e.g., Monaco Grand Prix) take place on closed streets in and around cities. The design of the F1 cars and rules of F1 racing are under the control of the Paris-based Fédération International de l'Automobile (FIA). The first Grand Prix auto race was held in France in 1906, but it was not until after World War II that F1 racing was born; it soon became one of the world's most popular—and most expensive—sports.

Today's Grand Prix races feature national teams and a standard racing circuit. F1 racecars are usually made by major automobile manufacturers called constructors—Ferrari, Porsche, Mercedes-Benz (DaimlerChrysler), Renault, Toyota, and others—and are maintained by full-time teams. The teams are usually sponsored by large corporations, often in cooperation with an automobile company; some are sponsored solely by car companies. F1 racing was traditionally centered in Europe, but F1 Grand Prix races are now held worldwide. Since 1950 the FIA has declared an annual F1 world champion constructor and driver. Among the best-known drivers are Argentina's Juan Fangio (1950s), Britain's Jackie Stewart (1960s-70s), America's Mario Andretti (1978), Austria's Niki Lauda (1970s-80s), France's Alain Prost (1980s-90s), and Germany's Michael Schumacher (1990s-2000s).

Sum of the atomic weights of all atoms in a chemical formula. The term is generally applied to a substance that consists of ions (see ionic bond) rather than individual molecules (and thus does not have a molecular weight). An example of such a substance is sodium chloride (table salt). Such a substance's chemical formula describes the simplest ratio of the number of atoms of the constituent elements. Seealso stoichiometry.

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Expression of the composition or structure of a chemical compound. Formulas for molecules use chemical symbols with subscript numbers to show the number of atoms of each element: O₂ for molecular oxygen, O₃ for ozone, CH₄ for methane, C₆H₆ for benzene. Parentheses may enclose atoms that act as a group. General formulas show the proportions of atoms in members of a class (e.g., C_math.nH_{2math.n+ 2} for alkanes). If the substance does not exist as molecules (see ionic bond), empirical formulas show the relative proportions of the constituents (e.g., NaCl for sodium chloride). Structural formulas show bonds (see bonding) between atoms in a molecule as short lines between symbols; they are particularly useful for showing how isomers differ. A projection formula also indicates the three-dimensional arrangement of the atoms (see Fischer projection; stereochemistry).

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