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Percent Composition, Empirical and Molecular Formulas

Percent Composition, Empirical and Molecular Formulas. Mass Percent. Percent of each element a compound is composed of. Find the mass of each element, divide by the total mass of the compound, then multiply by a 100. CH 4 Al 2 (Cr 2 O 7 ) 3 CaSO 4 · 2H 2 O. Empirical Formula.

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Percent Composition, Empirical and Molecular Formulas

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  1. Percent Composition, Empirical and Molecular Formulas

  2. Mass Percent • Percent of each element a compound is composed of. • Find the mass of each element, divide by the total mass of the compound, then multiply by a 100. • CH4 • Al2(Cr2O7)3 • CaSO4 · 2H2O

  3. Empirical Formula • From percent composition, you can determine the empirical formula. • Empirical Formula the lowest ratio of atoms in a molecule. • Based on mole ratios.

  4. Steps for Calculating Empirical Formula • 1. Calculate grams of each element. If % is given, assume 100 g sample. • 2. Convert grams to moles of each element using Molar Mass.

  5. 3. Divide by the lowest # of moles to get the Atomic Ratio. • 4. Round to the nearest whole number. • 5. Write the formula.

  6. Empirical Formula Example: • A sample is 59.53% C, 5.38%H, 10.68%N, and 24.40%O what is its empirical formula.

  7. Rounding to the Nearest Whole Number: • If the mole ratio comes out to be a fraction, multiply all ratios times the denominator of the fraction. • Example: 2.48 = 2 ½ therefore 2.48 x 2 = 5

  8. Empirical Formula Example 2: • A compound contains 26.56% Potassium, 35.41% chromium and the remainder is oxygen. Calculate the empirical formula.

  9. Molecular Formulas • Not the simplest Ratio. • To determine the molecular formula of a compound, you must be given the molecular mass.

  10. 1. Determine the empirical formula. • 2. Calculate the molar mass of the empirical formula • 3. • 4. Multiply all elements in the empirical formula by this factor.

  11. Example 1 • Find the molecular formula of a compound that contains 30.45% nitrogen and 69.55 % oxygen. The molar mass of the compound is 92.02 g/mol.

  12. Example 2 • Find the molecular formula of a compound that contains 56.36 g of oxygen and 43.64 g of phosphorus. The molar mass of the compound is 283.9 g/mol.

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