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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. - PowerPoint PPT Presentation
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Chapter 11-12
Empirical & Molecular Formula
The Empirical FormulaThe lowest whole number ratio of
elements in a compound.The molecular formula is the actual
ratio of elements in a compound.
CH2 empirical formulaC2H4 molecular formulaC3H6 molecular formula
Finding Empirical Formulas
Just find the lowest whole number ratio
C6H12O6
CH4N2It 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.
ExampleCalculate the empirical formula of a
compound composed of 38.67 % C, 16.22 % H, and 45.11 %N.
Assume 100 g so38.67 g C x 1mol C = 3.22
mole C 12.0 gC 16.22 g H x 1mol H = 16.1 mole
H 1.0 gH45.11 g N x 1mol N = 3.22 mole
N 14.0 gN
ExampleThe ratio is 3.22 mol C = 1 mol C 3.22 molN 1 mol N
The ratio is 16.1 mol H = 5 mol H 3.22 molN 1 mol N
C1H5N1
Your Turn
Calculate the empirical formula of a compound composed of 25.14 % C, 7.19 % H, and 67.67 %N.
Empirical to molecularempirical 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.
ExampleA compound has an empirical formula of
ClCH2 and a molar mass of 98.96 g/mol. What is its molecular formula?Cl = 35.5 g/mol X 1 = 35.5 g/molC = 12.0 g/mol X 1 = 12.0 g/molH = 1.0 g.mol X 2 = 2.0 g/mol
49.5 g/mol98.96 g/mol ÷ 49.5 g/mol = 22 (ClCH2) = Cl2C2H4
Gives us the Molecular Formula
Your TurnA compound has an empirical formula of CH2O and a molar mass of 180.0 g/mol. What is its molecular formula?
Percent to molecularTake the percent X the molar massThis gives you mass in one mole of the compound
Change this to molesYou will get whole numbersThese are the subscripts
ExampleCaffeine 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 194g= 96g ÷ 12.0g/mol = 85.15% H x 194g = 10g ÷ 1.0g/mol = 1028.87% N x 194g = 56g ÷ 14.0g/mol = 416.49% O x 194g = 32g ÷ 16.0g/mol = 2
C8H10N4O2 Molecular FormulaEmpirical Formula….C4H5N2O
Your TurnIbuprofen 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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