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Calculating Empirical Formula

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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 g S g O

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Sample Problem 7-12 Continued Calculate moles: g Na x 1 mole/22.99 g = g S x 1 mole/32.07 g = g O x 1 mole/16.00 g = Use moles as subscripts Na S O Divide by smallest subscript Na 1.408/ S / O 2.812/ Na S 1 O Round to get final answer: Na 2 SO 4

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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 P / O / P 1 O Multiply to get whole number ratio P 2 O 5

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