Lesson Objectives Calculate percent composition from a chemical formula

1. Lesson Objectives • Calculate percent composition from a chemical formula • Determine the empirical and molecular formulas for a substance • Percent Composition, Empirical, and Molecular Formulas

2. Percent composition describes the contribution of each element to the total mass of a compound • Calculations done for one mole of a substance • Ratio of one part of the compound to the whole compound • Percent Composition

3. To determine the percent composition, you need to know • Formula of the compound • Molar mass of each element in the compound • Molar mass of the compound Ex) Find the percent composition of CaCl2 • Calculating Percent Composition

4. Empirical formula – formula that shows the simplest whole number ratio of elements in a compound • Molecular formula – formula of one molecule or formula unit of a compound as it actually exists • Can be a multiple of the empirical formula Ex) N2O4 is a molecular formula with an empirical formula of NO2 • Sometimes the empirical and molecular formulas are the same Ex) NH3 is the molecular formula and the empirical formula of ammonia • Empirical and Molecular Formulas N H H H

5. Reduce a molecular formula to an empirical formula by dividing the subscripts by a factor that produces the lowest whole number ratio of subscripts Ex) C6H12 Empirical formula is CH2 Ex) H2SO4 Empirical formula is the same as the molecular, H2SO4 • Empirical Formula from Molecular Formula

6. Molecular formula can be a multiple of the empirical formula • Determine the molar mass of the empirical formula • Divide the molar mass of the molecular formula by the molar mass of the empirical formula to find the factor • Multiply the empirical formula by the factor Ex) What is the molecular formula of a compound with an empirical formula of CH2O and molar mass 180.18 g/mol? • Molecular Formula from Empirical Formula and Molar Mass

7. Percent composition gives a ratio of the masses of the elements in a compound • Convert masses into moles • Assume a 100 g sample • Divide each mole value by the smallest mole value • Whole number products are the subscripts for the empirical formula (see rounding rules) Ex) A compound is 82.8% carbon and 17.2% hydrogen. The molar mass is 58.14 g/mol. Find the empirical and molecular formulas. • Empirical and Molecular Formulas from Percent Composition and Molar Mass

8. If a number ends with a decimal of .01 or .99 then round to the nearest whole number • Sometimes the decimals will not be close enough to a whole number to round • Create whole numbers by multiplying all the mole values by the smallest possible factor • Look for these decimals during calculations • Ends in .5 • Equivalent of ½ • Multiply by 2 • Ends in .33 or .66 • Equivalents of or • Multiply by 3 • Ends in .25 or .75 • Equivalents of ¼ or ¾ • Multiply by 4 • Rounding in Empirical Formula Calculations Back to Problem

9. Percent composition gives a ratio of the masses of the elements in a compound • Convert masses into moles • Assume a 100 g sample • Divide each mole value by smallest mole value • Whole number products are the subscripts for the empirical formula • Divide the molar mass of the molecular formula by the molar mass of the empirical formula to find the factor • Multiply the empirical formula by the factor Ex) A compound is 82.8% carbon and 17.2% hydrogen. The molar mass is 58.14 g/mol. Find the empirical and molecular formulas. • Empirical and Molecular Formulas from Percent Composition and Molar Mass

10. Analysis of a chemical used in photographic developing fluid indicates a chemical composition of 65.45% C, 5.45% H, and 29.09% O. The molar mass is found to be 110.0 g/mol. Determine the molecular formula. • Practice Problem