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Understanding Empirical and Molecular Formulas: From Percent Composition to Ratios

This guide delves into the concepts of empirical and molecular formulas in chemistry. An empirical formula represents the lowest whole number ratio of elements in a compound, while a molecular formula shows the actual number of atoms. The relationship between the two can be illustrated through examples, such as determining the empirical formula from percent composition or converting grams to moles. We explain the steps to calculate the empirical formula and how to derive the molecular formula from it, highlighting key calculations and examples for clarity.

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Understanding Empirical and Molecular Formulas: From Percent Composition to Ratios

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  1. Empirical Formula From percentage to formula

  2. The Empirical Formula • The lowest whole number ratio of elements in a compound. • The molecular formula the actual ratio of elements in a compound. • The two can be the same. • CH2 empirical formula • C2H4 molecular formula • C3H6 molecular formula • H2O both

  3. Calculating Empirical • Just find the lowest whole number ratio • C6H12O6 • CH4N • It is not just the ratio of atoms, it is also the ratio of moles of atoms. • In 1 mole of CO2there is 1 mole of carbon and 2 moles of oxygen. • In one molecule of CO2 there is 1 atom of C and 2 atoms of O.

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

  5. 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 1mol C = 3.220 mole C 12.01 gC • 16.22 g H x 1mol H = 16.09 mole H 1.01 gH • 45.11 g N x 1mol N = 3.219 mole N 14.01 gN

  6. Example 3.220 mole C 16.09 mole H 3.219 mole N • The ratio is 3.220 mol C = 1 mol C 3.219 molN 1 mol N • The ratio is 16.09 mol H = 5 mol H 3.219 molN 1 mol N • C1H5N1

  7. Empirical to molecular • Since the empirical formula is the lowest ratio the actual molecule would weigh more. • By a whole number multiple. • Divide the actual molar mass by the the mass of one mole of the empirical formula.

  8. Example • Benzene has an empirical formula of CH It’s molecular mass is 78 g/mol. What is it’s molecular formula? • (Empirical mass)( X ) = Molecular Mass 13 X = 78 X = 6 • (CH) X = Molecular Formula (CH) 6 = C6H6

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