"A chromium compound contains 28.4% sodium and 32.1% chromium by mass, while the rest is oxygen. What is the empirical formula of this compound?"

Start with the definition of empirical formula so we know what we are trying to work out. Empirical formula is the simplest whole-number ratio of elements in a compound. First, let us work out how much of the compound is oxygen. As a percentage, the compound must contain 39.5% as 100 - 28.4 - 32.1 = 39.5%If we set it out in a table it may be easier to work out: Na Cr OMass 28.4 32.1 39.5Mr 23 52 16Mol 1.2 0.6 2.4 Ratio 2 : 1 : 4Therefore, empirical formula = Na₂CrO₄First, write out the mass by percentage of each component. In the next row, write each elements molecular weight. To determine the molecular ratio of elements we must find the number of moles of each element. We will do this by using the moles = mass/Mr equation. So for Na, we find the moles by 28.4/23 to get 1.2 moles. The numbers in the third row give the ratio of elements in the compound. However, the empirical formula definition stated that we need it to be a whole-number ratio so we must divide all the numbers by our smallest number. We keep repeating this until we get a whole number ratio. In this case, the empirical formula came to Na₂CrO_4.