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The molecule of methane can be represented as C2H6.
So, the molecular formula will be C2H6 and the empirical formula will be CH3.
The molecular mass can be found out as (2X12)+(6X1)=30u.
The empirical mass will be (1X12)+(3X1)=15u.
So, the difference between the molecular and empirical mass = 30u−15u=15u.