Unit 6: Chemical Quantities Chapter 10: Empirical and Molecular Formulas

Empirical Formulas Empirical Formulas The lowest whole-number ratio of atoms of the elements in a compound. Empirical Formulas are much like creating a recipe of elements. Orange Juice  1 can of concentrate to 3 cans of water = (OJ)1(H2O)3 = 1:3 ratio Double Batch (OJ)2(H2O)6 = 2:6 = 1:3 ratio

Empirical Formulas Steps in determining Empirical Formulas : Assume that the percent given is equal to the number of grams in a 100 gram sample. Meaning…remove the % and add g for the units. Convert the grams to moles using the molar mass. Divide each mole value by the smallest mole calculated to determine a ratio. If all the values are not near a whole number (_.8-_.2 range), multiply ALL the values by 2 or 3 until the values are all whole numbers. Give the Empirical Formula

Empirical Formulas Practice with Steps A compound is analyzed and found to contain 23.9 % nitrogen and 74.1 % oxygen. Find the empirical formula of the compound. Step 1: Assume percents are grams Nitrogen = 23.9% = 23.9g Oxygen = 74.1% = 74.1g

Empirical Formulas Practice with Steps A compound is analyzed and found to contain 23.9 % nitrogen and 74.1 % oxygen. Find the empirical formula of the compound. Step 2: Convert to moles using molar mass Nitrogen = 23.9 g 1 mol =1.85 mol Nitrogen 14 g Oxygen = 74.1 g 1 mol = 4.63 mol Oxygen 16 g

Empirical Formulas Practice with Steps A compound is analyzed and found to contain 23.9 % nitrogen and 74.1 % oxygen. Find the empirical formula of the compound. Step 3: Divide each mole by the smallest mole value Nitrogen = 1.85 mol ÷ 1.85 mol = 1mol of N Oxygen = 4.63 mol ÷ 1.85 mol = 2.5 mol of O

Empirical Formulas Practice with Steps A compound is analyzed and found to contain 23.9 % nitrogen and 74.1 % oxygen. Find the empirical formula of the compound. Step 4: Multiply the moles by a coefficient to have whole number ratios. Nitrogen = 1 mol x 2 = 2 mol of N Oxygen = 2.5 mol x 2 = 5 mol of O Step 5: Give Empirical Formula N2O5

Molecular Formulas Molecular Formulas Requires two things Based on Empirical Formulas Most molecular formulas are empirical formulas, but some molecular formulas are not the lowest ratios. Example: Hydrogen Peroxide Empirical Formula  HO Molecular Formula  H2O2 Requires two things Mass of the Compound Empirical Formula of the Compound

Molecular Formulas Practice Calculate the molecular formula of a compound whose mass is 60.0 g and empirical formula is CH4N. Find the molar mass of the empirical formula CH4 N = 30.0 g/mol

Molecular Formulas Practice Calculate the molecular formula of a compound whose mass is 60.0 g and empirical formula is CH4N. Divide the molecular mass by the empirical mass Molecular mass/ empirical mass (60.0 / 30.0 ) = 2

Molecular Formulas Practice Calculate the molecular formula of a compound whose mass is 60.0 g and empirical formula is CH4N. Multiply the empirical formula subscripts by the coefficient found in step 2. C (1x2) H (4x2) N (1x2) = C2H8N2