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**III. Formula Calculations**

The Mole III. Formula Calculations C. Johannesson

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**A. Percentage Composition**

the percentage by mass of each element in a compound C. Johannesson

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**A. Percentage Composition**

Find the % composition of Cu2S. g Cu g Cu2S %Cu = 100 = 79.852% Cu 32.07 g S g Cu2S %S = 100 = 20.15% S C. Johannesson

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**A. Percentage Composition**

Find the percentage composition of a sample that is 28 g Fe and 8.0 g O. 28 g 36 g %Fe = 100 = 78% Fe 8.0 g 36 g %O = 100 = 22% O C. Johannesson

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**A. Percentage Composition**

How many grams of copper are in a 38.0-gram sample of Cu2S? Cu2S is % Cu (38.0 g Cu2S)( ) = 30.3 g Cu C. Johannesson

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**A. Percentage Composition**

Find the mass percentage of water in calcium chloride dihydrate, CaCl2•2H2O? 36.04 g g %H2O = 100 = 24.51% H2O C. Johannesson

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**C2H6 CH3 B. Empirical Formula**

Smallest whole number ratio of atoms in a compound C2H6 reduce subscripts CH3 C. Johannesson

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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. C. Johannesson

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B. Empirical Formula Find the empirical formula for a sample of 25.9% N and 74.1% O. 25.9 g 1 mol 14.01 g = 1.85 mol N = 1 N 1.85 mol 74.1 g 1 mol 16.00 g = 4.63 mol O = 2.5 O C. Johannesson

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**N2O5 N1O2.5 B. Empirical Formula**

Need to make the subscripts whole numbers multiply by 2 N2O5 C. Johannesson

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**CH3 C2H6 C. Molecular Formula**

“True Formula” - the actual number of atoms in a compound CH3 empirical formula ? C2H6 molecular formula C. Johannesson

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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. C. Johannesson

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**(CH2)2 C2H4 C. Molecular Formula**

The empirical formula for ethylene is CH2. Find the molecular formula if the molecular mass is g/mol? empirical mass = g/mol 28.1 g/mol 14.03 g/mol = 2.00 (CH2)2 C2H4 C. Johannesson

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85.7% C Percent Composition 24g C 28g CH4 mass of element x 100 x 100

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