Here’s a general strategy for determining empirical formulas. QUESTION: A sample of a compound contains 0.5860 mol P and 1.466 mol O. What is the empirical formula of the compound? A. PO, B. PO4, C. P2O5, D. P3O4 General strategy: determine count-to-count (or mole-to-mole) ratio; reduce ratio to small whole numbers (first divide all counts by the smallest count; then, if necessary, find smallest multiplier to make all numbers very close to whole numbers) A sample of a compound contains 0.5860 moles of phosphorus and 1.466 moles of oxygen. What is the empirical formula of the compound? A. PO, B. PO4, C. P2O5, D. P3O4 PAUSE CLICK Here’s a general strategy for determining empirical formulas. You can determine empirical formulas if you have enough information to determine the relative numbers of the atoms of different elements in the compound. In other words, if you can get a count-to-count ratio for the atoms of the elements in the compound. Remember that moles are just group counts, so you can also use mole-to-mole ratio. Once you have determined the mole-to-mole ratio, all you have to do is to reduce the ratio to a small whole number ratio. Here’s a table that shows how this strategy is implemented. From the question, we note that the compound contains 0.5860 moles of phosphorus HIGHLIGHT number in question and in table and also contains 1.466 moles of oxygen So, the phosphorus-to-oxygen mole ratio is 0.5860 to 1.466. A good strategy for reducing this ratio to a whole number ratio is to first divide both numbers by the smaller number. HIGHLIGHT “first divide all counts by the smallest count” In this case, 0.5860 is the smaller number. Dividing 0.5860 by 0.5860 gives us 1.000 HIGHLIGHT 1.000 and dividing 1.466 by 0.5860 gives us 2.502. HIGHLIGHT 2.502 Note that we’re keeping four significant figures in the result because our original numbers have four significant figures. We can see here that 2.502 is almsot halfway between two whole numbers. When the decimal part is at least 0.1 away from a whole number, you should not just round it off. What we need to do at this point is find the smallest whole number multiplier that will make all the numbers close to a whole number. HIGHLIGHT “if necessary, find smallest multiplier to make all numbers very close to whole number” Try 2. If that doesn’t work. Try 3, and then 4, and so on.... OK. Let’s try multiplying both numbers by 2 1.000 times 2 equals 2.000 and 2.502 times 2 equals 5.004 Now, we have two numbers that are very close to whole numbers. Therefore, the phosphorus-to-oxygen ratio is “2 is to 5” The empirical formula is P2O5. The correct answer is choice C. CLICK PAUSE END RECORDING Phosphorus Oxygen 0.5860 1.466 1.000 2.502 2.000 5.004

Video ID: 4-7-4 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08