Chemical Substances

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Lecture 4-Chemical Substances and Mole concept

Chemical Substances

  • Formulae and names; representing of molecules chemical reactions, writing and balancing chemical equations, Mole concept

Formulae and names

  • What is a chemical formula?
  •  chemical formula (plural: formulae) is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule using chemical element symbols, numbers, and sometimes also other symbols.
  • The simplest types of chemical formulae are called empirical formulae, which use letters and numbers indicating the numerical proportions of atoms of each type. Molecular formulae indicate the simple numbers of each type of atom in a molecule, with no information on structure. 
  • Example: the empirical formula for glucose is CH2O, while its molecular formula is C6H12O6 .
  • Other examples: The empirical and molecular formula of acetic acid is: CH3COOH
  • The empirical and molecular formula of Aluminium hydroxide is: Al(OH)3
  • The empirical formula for Benzene is CH, while its molecular formula is C6H6 

Naming 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, “octa-” is eight, “nona-” is nine, and “deca” .
  • Molecular compounds are named with the first element first and then the second element by using the stem of the element name plus the suffix –ide.
  • In writing Chemical formula always we write the positive ion (cation) first then the negative ion (anion).
  • Example: NaCl , HI, H2SO4


  • Formula Name
  • NO nitrogen monoxide
  • N2O dinitrogen monoxide
  • S2Cl2 disulfur dichloride
  • Cl2O7 dichlorine heptoxide
  • For some simple covalent compounds, we use common names rather than systematic names.
  • H2O: water
  • NH3: ammonia
  • CH4: methane
  • H2O2: hydrogen peroxide

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