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Lecture 6 Empirical &amp; Molecular Formulas

# Lecture 6 Empirical &amp; Molecular Formulas - Lec-5...

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Lec-5: Empirical & Molecular Formulas Roy A. Lacey, Stony Brook University; Che 131, Spring 2011 1 Percentage Composition ` Mass percent of an element in a compound Mass % mass of element in compound 100% = × M Cl mass of compound MgCl 2 Mg: 1 x 24.3 = 24.3 g Cl: 2 x 35.4 = 70.8 g formula mass 95 1 g formula mass = 95.1 g % Mg: 24 3/95 1 x 100 = 25 6% Mg: 24.3/95.1 x 100 = 25.6% %Cl: 70.8/95.1 x 100 = 74.4% Note: 25 6% + 74 4% = 100% Roy A. Lacey, Stony Brook University; Che 131, Spring 2011 2 Note: 25.6% + 74.4% = 100%

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Question Find the percentage composition of iron in: iron (III) silicate Fe 2 (SiO 3 ) 3 Fe: 2 x 55.8 Si: 3 x 28.1 a) 32.9% b)24.8% c) 16 0% O: 9 x 16.0 ---------- 340 g c) 16.0% d)42.4% 340. g %Fe: 2 x 55.8 = 112./340. x 100 = 32.9% %Si: 3 x 28.1 = 84.3/340. x 100 = 24.8% %O: 9 x 16.0 = 144./340. x 100 = 42.4% Roy A. Lacey, Stony Brook University; Che 131, Spring 2011 3 Empirical Formula The empirical formula of a chemical compound is the simplest The empirical formula of a chemical compound is the simplest whole number (mole) ratio of atoms of each element present in that compound. Finding the empirical formula is somewhat the reverse of finding percentage composition Find the empirical formula for a compound whose
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