Example Problems Ch. 10.4

Percent Composition Example 1: Calculate the percent composition of oxygen in a molecule of water. Step 1:Find the molar mass of water. 18.02 g/mol Step 2: Use the “Mass Percent” equation to solve. Mass percent = 16.00 g/mol O ______________ X 100 = 88.79% O 18.02 g/mol H2O

Percent Composition Example 2: Calculate the percent composition of all elements in a molecule of HF (hydrofluoric acid). Step 1:Find the molar mass of HF. 20.00 g/mol Step 2: Use the “Mass Percent” equation to solve for each element. Mass percent = 1.008 g/mol H ______________ X 100 = 5.040% H 20.00 g/mol HF

Percent Composition Example 2: (continued) Mass percent = 18.99 g/mol F ______________ X 100 = 94.95% F 20.00 g/mol HF You can check your work by adding the two percents together. They should be close to 100%. 5.040% + 94.95% = 99.99%

Percent Composition Challenge Problem How many grams of aluminum could be produced from 30.0 g of AlCl3? Step 1: Solve for percent mass of aluminum. 20.24% Al Step 2: Multiply this percentage to the original mass of the compound. 30.0 g x .2024 = 6.072 g Answer: 6.072 g of aluminum could be produced.

NOW YOU DO IT! Complete the GREEN Percent Composition Worksheet in your groups. You will have 30 min. of class time to work on this. Whatever you do not finish is homework. The answer key is available to check your work in the answer key folder. GOOD LUCK!

Empirical Formula from % Composition Example 1: Calculate the empirical formula of a compound that is 75% carbon and 25% hydrogen? Step 1: Assume a 100 g sample of the compound. 75% C = 75g C 25% H = 25g H

Empirical Formula from % Composition Example 1: (continued) Step 2: Convert from grams of each element to moles of each element.

Empirical Formula from % Composition Example 1: (continued) Step 3: Divide both numbers by the smallest of the two. 6.2 mol C / 6.2 = 1.0 mol C 25 mol H / 6.2 = 4.0 mol H Step 4: Write the empirical formula. CH4

NOW YOU DO IT! Complete the PINK Determining Empirical Formulas Worksheet in your groups. You will have 15 min. of class time to work on this. Whatever you do not finish is homework. The answer key is available to check your work in the answer key folder. Notice: The first problem on this sheet has been done for you! GOOD LUCK!

Empirical Formula from Mass of Element Example 1: What is the empirical formula for a compound which contains 5.41g Fe, 4.64g Ti, and 4.65g O? Step 1: Convert from grams of each element to moles of each element.

Empirical Formula from Mass of Element Example 1: (continued) Step 2: Divide each number by the smallest of the three. .00969 mol Fe/.00969 = 1.00 mol Fe .00969 mol Ti /.00969 = 1.00 mol Ti .291 mol O /.00969 = 3.00 mol O Step 3: Write the empirical formula. FeTiO3

NOW YOU DO IT! Complete the YELLOW Empirical Formula Worksheet in your groups. You will have 15 min. of class time to work on this. Whatever you do not finish is homework. The answer key is available to check your work in the answer key folder. GOOD LUCK!

Molecular Formula Determine the molecular formula of a compound with an empirical formula of CH and a molar mass of 78.11 g/mol. Step 1: Calculate the empirical molar mass. 1 C= 12.01 g/mol 1 H= 1.008 g/mol + ___________ 13.02 g/mol

Molecular Formula (continued) Step 2: Calculate “n” using the molar mass and the empirical molar mass.

Molecular Formula (continued) Step 3: Multiply “n” by the empirical formula to get the molecular formula. 6(CH) = C6H6 Notice: In some problems, you may not be given the empirical formula. In these cases, you will have to calculate the empirical formula first, then follow the steps to solve for molecular formula.