We can determine the empirical formula of an unknown compound from experimental data. Using instrumentation in the lab, we can determine the elemental mass composition of a compound. In other words, we can determine the percent of each element present in the compound. From that information, we can backtrack to determine the empirical formula of an unknown compound.
Watch these videos, which go through these types of problems step by step.
Source: Khan Academy, https://youtu.be/XvFOUVA2lZE, https://youtu.be/eqAzqyZjc50
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License.