"why is it unnecessary to find the mass percent after converting the grams of CO2 and H2O into grams of C and H?"
In a question like this, you do use mass percents, in the beginning when you are converting the grams of CO2 and H2O to grams of C and H.
The steps to solve are
-Find mass percentage of carbon in CO2, multiply this by the given grams of CO2 to get how many grams of carbon are present.
-Find the mass percentage of hydrogen in H20, multiply this by the given grams of H20 to get how many grams of hydrogen are present.
-Convert the grams of carbon to moles.
-Convert the grams of hydrogen to moles.
-Divide each moles value by your smaller answer.
-You will get a ratio of C to H, telling you the empirical formula for the hydrocarbon, which is the objective of the question and your answer.