What is the empirical formula of a compound that contains 69.5% oxygen and 30.5% nitrogen?
Look at the percentages. Notice that they add up to 100%. So there is nothing but Nitrogen and Oxygen in this compound. Remember that the atomic mass of oxygen (16) is very close to the atomic mass of nitrogen (14). Notice that the percentage of Oxygen is about twice the percentage of Nitrogen. This means that there must be about twice as much Oxygen as there is Nitrogen in the compound. There are two compounds that have this ratio, Nitrogen dioxide (NO2), and Dinitrogen Tetroxide (N2O4). Here are wikipedia references to both: http://en.wikipedia.org/wiki/Nitrogen_dioxide http://en.wikipedia.org/wiki/Dinitrogen_tetroxide If you will take the molar mass of either compound from the above pages and you double check the percentages with a calculator, you will find that both of these compounds closely match your numbers in the question.
