A sample of a compound is 80% carbon and 20% hydrogen by mass. Its formula mass is 30 amu. What is the molecular formula?
As with all these problems, it is usually assumed that we have a
We divide the molar quantities thru by the SMALLER molar quantity:
And thus the
And we know that the molecular formula is always a multiple of the empirical formula, and thus is terms of mass:
But we have a molecular mass of
This is not a realistic problem, as few analysts would perform combustion on a liquid, and no analyst could perform combustion on a gas.