# Empirical Molecular Formula Chapter 11 12 The Empirical

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Empirical & Molecular Formula Chapter 11 -12

The Empirical Formula The lowest whole number ratio of elements in a compound. The molecular formula is the actual ratio of elements in a compound. CH 2 empirical formula C 2 H 4 molecular formula C 3 H 6 molecular formula

Finding Empirical Formulas Just find the lowest whole number ratio C 6 H 12 O 6 CH 4 N 2 It is not just the ratio of atoms, it is also the ratio of moles of atoms.

Calculating Empirical Formulas Means we can get ratio from percent composition. Assume you have a 100 g. The percentages become grams. Turn grams to moles. Find lowest whole number ratio by dividing everything by the smallest moles.

Example Calculate the empirical formula of a compound composed of 38. 67 % C, 16. 22 % H, and 45. 11 %N. Assume 100 g so 38. 67 g C x 1 mol C = 3. 22 mole C 12. 0 g. C 16. 22 g H x 1 mol H = 16. 1 mole H 1. 0 g. H 45. 11 g N x 1 mol N = 3. 22 mole N 14. 0 g. N

Example The ratio is 3. 22 mol C = 1 mol C 3. 22 mol. N 1 mol N The ratio is 16. 1 mol H = 5 mol H 3. 22 mol. N 1 mol N C 1 H 5 N 1

Your Turn Calculate the empirical formula of a compound composed of 25. 14 % C, 7. 19 % H, and 67. 67 %N.

Empirical to molecular empirical formula = lowest ratio the actual molecule would weigh the same or more. Divide the actual molar mass by the molar mass of the empirical formula. Get a whole number. Multiply the empirical formula by this.

Example A compound has an empirical formula of Cl. CH 2 and a molar mass of 98. 96 g/mol. What is its molecular formula? Cl = 35. 5 g/mol X 1 = 35. 5 g/mol C = 12. 0 g/mol X 1 = 12. 0 g/mol H = 1. 0 g. mol X 2 = 2. 0 g/mol 49. 5 g/mol 98. 96 g/mol ÷ 49. 5 g/mol = 2 2 (Cl. CH 2) = Cl 2 C 2 H 4 Gives us the Molecular Formula

Your Turn A compound has an empirical formula of CH 2 O and a molar mass of 180. 0 g/mol. What is its molecular formula?

Percent to molecular Take the percent X the molar mass This gives you mass in one mole of the compound Change this to moles You will get whole numbers These are the subscripts

Example Caffeine is 49. 48% C, 5. 15% H, 28. 87% N and 16. 49% O. It has a molar mass of 194 g. What is its molecular formula? 49. 48% C x 194 g = 5. 15% H x 194 g = 28. 87% N x 194 g = 16. 49% O x 194 g = C 8 H 10 N 4 O 2 96 g ÷ 12. 0 g/mol = 8 10 g ÷ 1. 0 g/mol = 10 56 g ÷ 14. 0 g/mol = 4 32 g ÷ 16. 0 g/mol = 2 Molecular Formula Empirical Formula…. C 4 H 5 N 2 O

Your Turn Ibuprofen is 75. 69 % C, 8. 80 % H, 15. 51 % O, and has a molar mass of about 207 g/mol. What is its molecular formula?

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