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IIIIII III. Formula Calculations (p ) Ch. 3 & 7 – The Mole

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A. Percentage Composition n the percentage by mass of each element in a compound

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100 = A. Percentage Composition %Cu = g Cu g Cu 2 S 100 = %S = g S g Cu 2 S % Cu 20.15% S n Find the % composition of Cu 2 S.

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%Fe = 28 g 36 g 100 = 78% Fe %O = 8.0 g 36 g 100 = 22% O n Find the percentage composition of a sample that is 28 g Fe and 8.0 g O. A. Percentage Composition

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n How many grams of copper are in a 38.0-gram sample of Cu 2 S? (38.0 g Cu 2 S)( ) = 30.3 g Cu Cu 2 S is % Cu A. Percentage Composition

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100 = %H 2 O = g g 24.51% H 2 O n Find the mass percentage of water in calcium chloride dihydrate, CaCl 2 2H 2 O? A. Percentage Composition

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B. Empirical Formula C2H6C2H6 CH 3 reduce subscripts n Smallest whole number ratio of atoms in a compound

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B. Empirical Formula 1. Find mass (or %) of each element. 2. Find moles of each element. 3. Divide moles by the smallest # to find subscripts. 4. When necessary, multiply subscripts by 2, 3, or 4 to get whole #s.

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B. Empirical Formula n Find the empirical formula for a sample of 25.9% N and 74.1% O g 1 mol g = 1.85 mol N 74.1 g 1 mol g = 4.63 mol O 1.85 mol = 1 N = 2.5 O

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B. Empirical Formula N 1 O 2.5 Need to make the subscripts whole numbers multiply by 2 N2O5N2O5

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C. Molecular Formula n True Formula - the actual number of atoms in a compound CH 3 C2H6C2H6 empirical formula molecular formula ?

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C. Molecular Formula 1. Find the empirical formula. 2. Find the empirical formula mass. 3. Divide the molecular mass by the empirical mass. 4. Multiply each subscript by the answer from step 3.

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C. Molecular Formula n The empirical formula for ethylene is CH 2. Find the molecular formula if the molecular mass is 28.1 g/mol? 28.1 g/mol g/mol = 2.00 empirical mass = g/mol (CH 2 ) 2 C 2 H 4

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