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Published byMichelle Kerr Modified over 2 years ago

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Percent Composition The relative amounts of elements in a compound. % composition = mass of element mass of compound x g C 28g CH 4 x 100 = 85.7% C

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Percent Composition from Formula C2H4C2H4 2 moles of C x 12 g/mol = 24 g 4 moles of H x 1 g/mol = 4 g 28 g 24g/28g x 100 = 85.7% C 4g/ 28g x 100 = 14.3% H

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You try one: NaC 2 H 3 O 2

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Empirical Formula The lowest whole number ratio of elements in a compound

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1 st - Begin by pretending to have 100 g sample & convert the % to grams. 2 nd - Convert grams to the moles by dividing by molar mass. 3 rd - Obtain whole number ratios of moles by: 1) dividing each number by the smallest number 2) multiplying to get rid of any fractions 4 th - Write the empirical formula using the number of moles as subscripts. Calculate empirical formula from percent composition:

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Example 1 Calculate the empirical formula of a compound composed of % C, % H, and %N. Assume 100 g so g C / 12 g/mol C = 3.22 mole C g H / 1.07g/mol H = mole H g N / 14 g/mol N = 3.22 mole N Divide each by the smallest value

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3.220 mole C / 3.22 = mole H / 3.22 = mole N / 3.22 = 1 The empirical formula is CH 5 N Example 1 cont.

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A compound is % P and % O. What is the empirical formula? 43.6 g P /31 g/mol = 1.4 mole P /1.4 = g O /16 g/mol = 3.5 mole O /1.4 = 2.5 Multiply the result to get rid of any fractions. P 1 O X = P 2 O 5 Divide by smallest number Example 2

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Molecular Formulas Are whole number multiples of empirical formulas. 1)Divide molar mass of compound by molar mass of empirical formula. 2)Multiply subscripts in compound by the number from step 1. Ex. An empirical formula is CH 5 N. The molar mass is 93 g/mol. 93 g/mol / 31 g/mol = 3 3 x CH 5 N = C 3 H 15 N 3

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