Empirical Formulas

Empirical Formulas Empirical Formula: consists of the symbols for the elements combined in a compound, with subscripts of the smallest whole number ratio of the different atoms in the compound. Example Sugar has the formula of C6H12O6 Its empirical formula is CH2O

Empirical Formulas What is the empirical formulas for the following compounds? Na2C2O4 C3H6 K2SO4 B2H6

Empirical Formulas You can calculate the empirical formula of a compound from its percent composition. Example: You know a compound is 78.10 % B and 21.90 % H. What is the compounds empirical formula?

Empirical Formulas Step 1: Assume you have a 100 g sample so the percentages change to grams. 78.10 % B is now 78.10 g B 21.90 % H is now 21.90 g H

Empirical Formulas Step 2: Convert the grams to moles using the mole highway. 78.10 g B 1 mol B = 7.224 mol B 10.811 g B 21.90 g H 1 mol H = 21.73 mol H 1.00794 g H

Empirical Formulas Step 3: Divide each number by the smallest number to get a ratio. Round to the nearest whole or half number 7.224 is the smallest number 7.224/7.224 = 1 B 21.73/7.224 = 3.01  3 H

Empirical Formulas Step 4: If you have any half numbers, double all the numbers. If not move on.

Empirical Formulas Step 5: The numbers are the ratio of each element. Use them to write the formula. 1 B and 3 H makes BH3

Empirical Formulas Lets say we have a compound that is 26.56 % K, 35.45 % Cr and 38.03 % O.

Empirical Formulas Step 1: Assume you have a 100 g sample so the percentages change to grams. 26.56 % K is now 26.56 g K 35.41 % Cr is now 35.41 g Cr 38.03 % O is now 38.03 g O

Empirical Formulas Step 2: Convert the grams to moles using the mole highway. 26.56 g K 1 mol K = .6793 mol K 39.0983 g K 35.41 g Cr 1 mol Cr = .6810 mol Cr 51.9961 g Cr 38.03 g O 1 mol O = 2.377 mol O 15.9994 g O

Empirical Formulas Step 3: Divide each number by the smallest number to get a ratio. Round to the nearest whole or half number .6793 is the smallest number .6793/.6793 = 1 K .6810/.6793 = 1.003  1 Cr 2.377/.6793 = 3.499  3.5 O

Empirical Formulas Step 4: If you have any half numbers, double all the numbers. If not move on. 1 K x 2 = 2 K 1 Cr x 2 = 2 Cr 3.5 O x 2 = 7 O

Empirical Formulas Step 5: The numbers are the ratio of each element. Use them to write the formula. 2 K, 2 Cr and 7 O makes K2Cr2O7

Empirical Formulas Try the following problems: Find the empirical formula for a compound that is 32.38 % Na, 22.65 % S and 44.99 % O. Find the empirical formula for a compound that is 43.67 % P and 56.33 % O.

Empirical Formulas 1. Na2SO4 2. P2O5