• How about water? The largest whole number that will divide
evenly into 2 and 1 is 1. So the empirical formula for water is the
same as its molecular formula: H 2 O.- Percent Composition: This refers to the percent of a
substancecontained within a molecule. The test writers may ask you to
determine this percent. Here is one example of a question you might see on
the test.
Percent by Mass
Whenever a question asks
about the percent composition,
the implication is
the percent by mass of the
substance.What is the percent of oxygen by mass in hydrogen peroxide, H 2 O 2?
In order to determine the percent composition of oxygen, you need to find the
mass in amu of all of the oxygen atoms in the molecule and compare it to the
total formula weight. As shown earlier, each of the two oxygen atoms in H 2 O 2
weighs 16 amu. This means that the mass due to oxygen is 2(16) or 32 amu. The
formula weight of H 2 O 2 is 34 amu. So the percent of oxygen by mass in H 2 O 2 is
×100%, or roughly 94%. You can then calculate the percent of hydrogen
by mass in H 2 O 2 , which is 100% − 94%, or about 6%.