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Calculating Empirical Formula

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Empirical Formula Tells the simplest whole number molar ratio of elements in a compound

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**To Calculate Empirical Formula**

Assume 100 g of compound Convert percent composition of each element to grams Find the number of moles of each element Use moles as subscripts for formula Find the simplest whole number ratio by dividing all subscripts by smallest subscript

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Sample Problem 7-12 A compound contains 32.38% sodium, 22.65% sulfur, and 44.99% oxygen. What is the empirical formula? Assume 100 g of the compound and convert each percent to grams: 32.38g Na 22.65 g S 44.99 g O

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**Sample Problem 7-12 Continued**

Calculate moles: 32.38 g Na x 1 mole/22.99 g = 1.408 22.65 g S x 1 mole/32.07 g = 44.99 g O x 1 mole/16.00 g = 2.812 Use moles as subscripts Na1.408S0.7063O2.812 Divide by smallest subscript Na1.408/0.7063S0.7063/0.7063O2.812/0.7063 Na1.993S1O3.981 Round to get final answer: Na2SO4

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Sample Problem 7-13 Analysis of a g sample of a compound that contains only phosphorous and oxygen indicates the phosphorous content is g. What is the empirical formula? Find the mass of oxygen: g – g = g O Find the moles: P: g x 1 mole/30.97 g = mole P O: g x 1 mole/16.00 g = mole O

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**Sample Problem 7-13 Continued**

Use moles as subscripts: P O Divide by smallest mole P0.1431/ O /0.1431 P1O2.497 Multiply to get whole number ratio P2O5

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