NumericalModerate1 mark
7(iii)
Calculate:
Write the empirical formula of C₈H₁₈
Topic: Mole concept, gas volumes and stoichiometry
Previous-year question practice database
Answer
Exam answer
C₈H₁₈
The empirical formula is the simplest whole-number ratio of the atoms.
C : H = 8 : 18
Divide both by the highest common factor, 2: C : H = 4 : 9
Empirical formula = C₄H₉
Explanation
Find the highest common factor of the subscripts and divide both by it. Here the HCF of 8 and 18 is 2, giving C₄H₉.
Do not stop at the first factor that works but is not the largest, and do not force the ratio further — 4 and 9 share no factor, so C₄H₉ is as simple as it goes. Note the empirical formula need not be a real, stable molecule; it is only a ratio. The actual substance here is octane, C₈H₁₈.
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