Empirical Formulas

Reading activity With a partner take turns reading pages 241-246 Discuss the relationships between the following words and create a small word web to show how they are related Molecular Formula Empirical Formula Percent Composition Mole Molar Mass “n”

Definition Empirical Formulas of a chemical compound is shows the smallest whole number ratio of elements in a compound. Standard for ionic compounds

Where does it come from? The term empirical refers to a process of elemental analysis used to determine relative percent composition of a pure chemical substance by element.

Molecular Formula Molecular Formula identifies the absolute number of atoms of such elements to be found in each discrete molecule of that compound.

Examples Compound Empirical Molecular Multiplier Water H2O 1 Hydrogen peroxide HO H2O2 2 Propylene CH2 C3H6 3 Glucose CH2O C6H12O6 6 Vitamin C C3H4O3 C6H8O6

You may be given the following info: The masses of the elements in a weighed sample of the compound The mass percents of the element in the compound Assume a 100-g sample and calculate the masses of each element in that sample

From the information given… Find the number of moles of each element. (Gram to mole conversion usually) Find the simplest mole ratio and set that to 1. Find all other mole ratios. Apply the mole ratios as subscripts for your simple formula. If a mole ratio is X 0.5, then multiply ALL mole ratios by 2.

#1 0.6884gPb = 3.322*10-3 0.2356gCl = 6.6646*10-3 Mass of Pb Mass of Cl

Divide by the smallest number Pb = 1.000 mol Pb Cl: = 2.001 mol Cl ANSWER: PbCl2 Divide by the smallest number

#2 HINT: Assume 100 grams Answer: C6NH11O Answer #3: C3H3O

Molecular Formulas 71.65 g Cl = 2.021mol Cl 24.27 g C = 2.021mol C 4.07 g H = 4.04mol H Ratio: ClCH2

The molar mass of ClCH2 is 49.48 g (empirical formula mass) = = 2 ; thus the Molecular Formula is (ClCH2)2 or Cl2C2H4

Practice Problems Complete on the back or new page in your notebook