Your acid's molecular formula is
Since you're dealing with an organic acid, and since the only products formed when it undergoes combustion are carbon dioxide and water, you know that it contains three elements: carbon, hydrogen, and oxygen.
The carbon dioxide will contain all the carbon the inital sample contained. At the same time, the water will contain all the hydrogen, which means that you can use the amounts produced to get the mass of carbon and hydrogen in the sample.
Since you know the total mass of the sample, this will allow you to calculate the mass of oxygen as well.
Determine the percent composition of carbon in
This means that, for every 100 g of
Therefore, the initial sample contained
The mass of oxygen in the initial sample will thus be
Determine how many moles of each element you get in that much mass
Divide all these numbers by the smallest one to get the mole ratios of the elements within the compound
Since the empirical formula cannot contain fractional subscripts, multiply all the subscripts by 2 to get
To get the molecular formula, use the acid's molar mass and the molar mass of its empirical formula
Thus, the acid's molecular formula will be