A Level Chemistry OCR Practice Exam

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What is the general formula for alkanes?

CnH2n
CnH2n-2
CnH2n+2
CnH2n+1OH

The correct answer is: CnH2n+2

The general formula for alkanes is represented as CnH2n+2, where 'n' stands for the number of carbon atoms in the molecule. Alkanes are saturated hydrocarbons, meaning they contain the maximum number of hydrogen atoms bonded to carbon, without any double or triple bonds present. This structure allows each carbon atom to form single bonds with hydrogen atoms, filling its four bonding sites. For every carbon atom in an alkane, two additional hydrogen atoms are added to maintain this saturated structure. Therefore, for every increment in the number of carbon atoms (n), the number of hydrogen atoms increases by two more than twice the number of carbon atoms, which is encapsulated in the formula CnH2n+2. This distinction is important in organic chemistry, as it sets alkanes apart from unsaturated hydrocarbons, which would have different formulas such as CnH2n for alkenes (which contain double bonds) or CnH2n-2 for alkynes (which contain triple bonds). The presence of -OH groups in D (an alcohol) and the inconsistent hydrogen counts in the other options further clarify that these are not relevant to the structure of alkanes.