molecular formula

(noun)

A formula that describes the exact number and type of atoms in a single molecule of a compound.

Related Terms

Examples of molecular formula in the following topics:

  • Molecular Formulas

    • For example, for butane, the molecular formula is C4H10.
    • For example, the molecular formula of sodium fluoride is NaF.
    • Molecular formulas are more limiting than chemical names and structural formulas.
    • The molecular formula for glucose is C6H12O6.
    • To convert between empirical and molecular formulas, the empirical formula can be multiplied by a whole number to reach the molecular formula.

  • A Structure Formula Relationship

    • Recall that the molecular formula of a hydrocarbon (CnHm) provides information about the number of rings and/or double bonds that must be present in its structural formula.
    • In the formula shown below a triple bond is counted as two double bonds.
    • This molecular formula analysis may be extended beyond hydrocarbons by a few simple corrections.
    • All halogens present in the molecular formula must be replaced by hydrogen.
    • Each nitrogen in the formula must be replaced by a CH moiety.

  • Alkenes & Alkynes

    • The molecular formulas of these unsaturated hydrocarbons reflect the multiple bonding of the functional groups:
    • As noted earlier in the Analysis of Molecular Formulas section, the molecular formula of a hydrocarbon provides information about the possible structural types it may represent.
    • For example, consider compounds having the formula C5H8.
    • The formula of the five-carbon alkane pentane is C5H12 so the difference in hydrogen content is 4.

  • Empirical Formulas

    • The molecular formula for a compound is equal to, or a whole-number multiple of, its empirical formula.
    • What is the molecular formula of decane?
    • Step 1: Calculate the molecular weight of the empirical formula (the molecular weight of C = 12.011 g/mol and H = 1.008 g/mol)
    • Step 2: Divide the molecular weight of the molecular formula by the the molecular weight of the empirical formula to find the ratio between the two.
    • This video shows how to go from the molecular formula of a compound to its corresponding empirical formula.

  • Analysis of Molecular Formulas

    • Although structural formulas are essential to the unique description of organic compounds, it is interesting and instructive to evaluate the information that may be obtained from a molecular formula alone.
    • Consider a hydrocarbon with a molecular structure consisting of a simple chain of four carbon atoms, CH3CH2CH2CH3.
    • The molecular formula is C4H10 (the maximum number of bonded hydrogens by the 2n + 2 rule).
    • From the above discussion and examples it should be clear that the molecular formula of a hydrocarbon (CnHm) provides information about the number of rings and/or double bonds that must be present in its structural formula.
    • All halogens present in the molecular formula must be replaced by hydrogen.

  • Percent Composition of Compounds

    • The atomic composition of chemical compounds can be described using a variety of notations including molecular, empirical, and structural formulas.
    • 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 the entire compound.
    • For example, butane has a molecular formula of C4H10.
    • However, this information is insufficient to determine the molecular formula without additional information on the compound's molecular weight.
    • Translate between a molecular formula of a compound and its percent composition by mass

  • Polyatomic Molecules

    • The science of molecules is called molecular chemistry or molecular physics, depending on the focus.
    • Molecular chemistry deals with the laws governing the interaction between molecules resulting in the formation and breakage of chemical bonds; molecular physics deals with the laws governing their structure and properties.
    • A compound's empirical formula is the simplest integer ratio of its constitutional chemical elements.
    • The molecular formula characterizes different molecules by reflecting their exact number of compositional atoms.
    • The empirical formula is often the same as the molecular formula, but not always; for example, the molecule acetylene has molecular formula C2H2, but the simplest integer ratio of elements is CH.

  • High Resolution Spectra

    • By designing mass spectrometers that can determine m/z values accurately to four decimal places, it is possible to distinguish different formulas having the same nominal mass.
    • Mass spectrometry therefore not only provides a specific molecular mass value, but it may also establish the molecular formula of an unknown compound.
    • Since a given nominal mass may correspond to several molecular formulas, lists of such possibilities are especially useful when evaluating the spectrum of an unknown compound.

  • Molecules

    • Molecular size varies depending on the number of atoms that make up the molecule.
    • Often, a compound's composition can also be denoted by an empirical formula, which is the simplest integer ratio of its constituent chemical elements.
    • However, this empirical formula does not always describe the specific molecule in question, since it provides only the ratio of its elements.
    • The full elemental composition of a molecule can be precisely represented by its molecular formula, which indicates the exact number of atoms that are in the molecule.
    • The chemical formula for propanol (C3H7OH) describes several different molecules, which vary by the position of the alcohol (OH).

  • Naming Molecular Compounds

    • Molecular compounds are named using a systematic approach of prefixes to indicate the number of each element present in the compound.
    • Molecular compounds are made when two or more elements share electrons in a covalent bond to connect the elements.
    • Typically, non-metals tend to share electrons, make covalent bonds, and thus, form molecular compounds.
    • When naming molecular compounds prefixes are used to dictate the number of a given element present in the compound. " mono-" indicates one, "di-" indicates two, "tri-" is three, "tetra-" is four, "penta-" is five, and "hexa-" is six, "hepta-" is seven, "octo-" is eight, "nona-" is nine, and "deca" is ten.
    • This video explains how to use a chemical name to write the formula for that compound.