One of the most common mistakes that I've come across regards the general formulae of hydrocarbons.
Let's start with the general formulae of the following homologous series:
Alkanes: #C_nH_"2n+2"# - this means that for every carbon atom there will be double the number of hydrogen atoms +2 (for example, Methane is CH4 - we get this by doubling the number of carbon atoms and adding two.
Alkenes: #C_nH_2n# - this means that for every carbon atom there is double the number of hydrogen atoms (for example, Ethene is #C_2H_4#). Remember that a carbon atom has four valence electrons and, as such, can only partake in a maximum of four covalent bonds. Alkenes have a double bond present (represented as C=C) this means that there will be two less hydrogen atoms in the molecule: if two bonds are already taken, then where can the hydrogen go?
Alkynes: #C_nH_"2n-2"# - this means that for every carbon atom, there is double the number of hydrogen atoms -2. (for example, Ethyne is #C_2H_2#). The characteristic bond which defines an Alkyne is a triple bond. This triple bond means that three previously valence electrons have now bonded with those of another carbon atom. This leaves only one other potential place for hydrogen to bond - and if the triple bond is in the middle of the molecule, there is no opportunity for hydrogen to bond at all because three bonds will be between one other carbon atom, and one other bond would be with another one.
Cycloalkanes: #C_nH_2n# - this is the same general formula as the Alkenes. So, as with the Alkenes, for every carbon atom there is double the number of hydrogen atoms (for example, Cyclobutane is #C_4H_8#). The carbon atoms are arranged in a cyclic structure, so at least two bonds will take place with other carbon atoms. This leaves two free valence electrons which can bond with hydrogen. The best way to remember the naming of the Cycloalkanes is by remembering the corresponding shape to the number of carbon atoms (Cyclopentane has 5 carbon atoms and is shaped like a pentagon, for example).
