# Percent Composition Empirical and Molecular Formulas Mass Percent

• Slides: 12

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 –Ca. SO 4 · 2 H 2 O

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.

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.

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

Empirical Formula Example: • A sample is 59. 53% C, 5. 38%H, 10. 68%N, and 24. 40%O what is its empirical formula.

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

Empirical Formula Example 2: • A compound contains 26. 56% Potassium, 35. 41% chromium and the remainder is oxygen. Calculate the empirical formula.

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

• 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.

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.

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.