The molecular mass is C2H7
you have 186.5 g of the substance, so calculate
2(12.01)+7(1.01)=31.09
so 186.5/31.09=6
so you can say C12H42
but when you divide 2 by 2 and 7 by 2 you obtain 1 and 3.5 because you have 3.5 you have to multiplied by 2 again, so you obtain the same formula as the empirical formula.
It example is the same as water, which empirical formula es H2O as its molecular formula
To determine the molecular formula, you need to compare the molar mass of the empirical formula with the given molar mass. The molar mass of the empirical formula C2H7 is 29 g/mol. Divide the given molar mass (186.5 g) by the molar mass of the empirical formula to find the molecular ratio. In this case, the molecular formula is C6H21.