Question

A compound is found to be 40.0% carbon, 6.7% hydrogen, and 53.5% oxygen. Its molecular mass is 60. g/mol. What is its molecular formula?

Answer 1

The question gives you the percent mass of each element in the compound. For instance, carbon represents 40% of the molar mass, so of the 60 g/mol, 24 g/mol is contributed by carbon (60*0.4). You know that carbon has a molar mass of 12 g/mol, meaning you must have 2 carbon atoms present to get 24 g/mol.

Use this strategy to determine the number of hydrogen and oxygen atoms in the compound, and then you'll have your molecular formula :)