A Level Chemistry OCR Practice Exam

Which is the general formula for alkanes?

1. CnH2n

2. CnH2n+1

3. CnH2n+2

4. CnH2n-1

The correct answer is: CnH2n+2

The general formula for alkanes is represented as CnH2n+2. This formula indicates that for every n carbon atoms in the molecule, there are 2n + 2 hydrogen atoms. Alkanes are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms and are fully "saturated" with hydrogen, which maximizes the number of hydrogen atoms that can bond to each carbon atom. Understanding this formula is crucial because it helps in identifying the structure and the number of hydrogen atoms associated with various alkane compounds. For example, if you take methane (which is the simplest alkane with one carbon atom), substituting n with 1 gives C1H(2(1)+2), resulting in CH4, which is indeed methane. The other options do not represent alkanes. For instance, CnH2n represents alkenes, which contain at least one double bond and are therefore unsaturated. CnH2n+1 implies one hydrogen atom less than an alkane, typically found in alkyl groups derived from alkanes. CnH2n-1 suggests a compound with fewer hydrogens than would be present in a fully saturated alkane, which is not