H2O2
First, let's determine how many moles of hydrogen and oxygen atoms we have. Start by looking up the atomic weights of those elements:
Atomic weight hydrogen = 1.00794
Atomic weight oxygen = 15.999
Moles hydrogen = 1.33 g / 1.00794 g/mol = 1.319522987 mol
Moles oxygen = 21.3 g / 15.999 g/mol = 1.331333208 mol
We now have a ratio of 1.319522987 : 1.331333208 and we want a ratio of small integers that is close. Start by dividing all the numbers in the ratio by the smallest value, giving:
1 : 1.008950371
This ratio is acceptably close to 1:1 so I assume the formula is of the form HnOn where n is a small integer. Let's initially assume that n is 1, so the mass would be
1.00794 + 15.999 = 17.00694
Obviously 17 is far smaller than 34.1. So let's divide 34.1 by 17.00694 and see what n should be:
34.1 / 17.00694 = 2.005063815
So the formula we want is H2O2, which is hydrogen peroxide.
