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 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 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 % oxygen. The molar mass of the compound is g/mol.

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