Here’s a general strategy for determining empirical formulas. QUESTION: A sample of a compound contains 19.27 g Fe and 7.360 g O. What is the empirical formula of the compound? A. FeO, B. FeO2, C. Fe2O3, D. Fe3O4 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 19.27 g Fe and 7.360 g O. What is the empirical formula of the compound? A. FeO, B. FeO2, C. Fe2O3, D. Fe3O4 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. Since we’re given masses in this question.... the first thing we need to do is convert them to moles By now you should know how to do this. Here’s a quick review We start with known masses of iron and oxygen then multiply CLICK CLICK by a conversion factor based on the molar mass CLICK CLICK CLICK CLICK CLICK CLICK 55.85 grams per mole is the molar mass of iron and 16.00 grams per mole is the molar mass of oxygen These calculations give us... 0.3450 moles of iron CLICK and 0.4600 moles of oxygen CONTINUED ON NEXT SLIDE 1 mol Fe 19.27 g Fe x = 0.3450 mol Fe 55.85 g Fe 1 mol O 7.360 g O x = 0.4600 mol O 16.00 g O

So, the iron-to-oxygen mole ratio is 0.3450 to 0.4600. QUESTION: A sample of a compound contains 19.27 g Fe and 7.360 mol O. What is the empirical formula of the compound? A. FeO, B. FeO2, C. Fe2O3, D. Fe3O4 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) So far, we have determined that the sample contained 0.3450 moles of iron HIGHLIGHT 0.3450 in table and 0.4600 moles of oxygen HIGHLIGHT 0.4600 in table So, the iron-to-oxygen mole ratio is 0.3450 to 0.4600. 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, 0.3450 is the smaller number. Dividing 0.3450 by 0.3450 gives us 1.000 HIGHLIGHT 1.000 and dividing 0.4600 by 0.3450 gives us 1.333. HIGHLIGHT 1.333 Note that we’re keeping four significant figures in the result because our original numbers have four significant figures. When the decimal part is at least 0.1 away from a whole number, you should not just round it off. We should not just round off 1.333 to a whole number. 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 CLICK and 1.333 times 2 equals 2.666 2.666 is still too far from a whole number. So let’s try multiplying the numbers by 3. 1.000 times 3 equals 3.000 and 1.333 times 3 equals 3.999 CLICK Now, we have two numbers that are very close to whole numbers. 3.999 is essentially equal to 4. Therefore, the iron-to-oxygen ratio is “3 is to 4” The empirical formula is Fe3O4. The correct answer is choice D. CLICK PAUSE END RECORDING Iron Oxygen 0.3450 mol 0.4600 mol 1.000 1.333 2.000 2.666 3.000 3.999

Video ID: 4-7-5 © 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