Percent Composition, Empirical and Molecular Formulas

What Is Percent Composition? A relative measure of the mass of each different element present in a compound.  Obviously, it is expressed as a percent (%).  It can be determined regardless of the # of elements in the compound.  The sum of all the % values MUST equal 100.

Why Is % Comp Important? Sometimes it is important to know what percent of a compound’s mass is due to a certain element.  A mining company may wish to know what % of 400 tons of Fe 2 O 3 is due to iron. They can then work out if it is worth trying to recover the iron from the sample of iron oxide.

STEPS TO Calculate % Comp 1. Calculate the mass of an element 2. Calculate the molar mass of the compound. 3. Divide the mass of the element by the molar mass of the substance. 4. Multiply by 100 to make it a percentage.

Calculating % Composition Calculate the % composition of magnesium carbonate, MgCO 3 Molar mass of magnesium carbonate: g g + 3(16.00 g) = g

Percent Composition Percent = mass of element in 1 mole x 100 by mass molar mass of compound

Let’s try some 1. What is the percent composition by mass of carbon in carbon dioxide (CO2)? C = x 1 atom = g O = x 2 atoms = g Total Mass = g Percent comp of carbon = g x 100% g = % of carbon

2. What is the percent composition of both hydrogen and oxygen in water? H = x 2 atoms = g O = x 1 atoms = g Total Mass = g Percent Comp H: 2.016/ = % Percent Comp O: / = % Your percentage for the entire compound should always add to 100 percent!

Let’s practice Complete Percent Comp Worksheet Complete Percent composition POGIL

Determining Formulas from Percent Composition

Definitions of Formulas  molecular formula = C 6 H 6 = (CH) 6  empirical formula = CH Empirical formula: the lowest whole number ratio of atoms in a compound. Molecular formula: the true number of atoms of each element in the formula of a compound.

Formulas (continued) Formulas for ionic compounds are ALWAYS empirical (lowest whole number ratio). Examples: NaCl MgCl 2 Al 2 (SO 4 ) 3 K 2 CO 3

Formulas (continued) Formulas for covalent (molecular) compounds MIGHT be empirical (lowest whole number ratio). Molecular: H2OH2O C 6 H 12 O 6 C 12 H 22 O 11 : Empirical: H2OH2O CH 2 OC 12 H 22 O 11

Let’s start with Calculating Empirical Formulas from Percent Composition!

Empirical Formula Determination In the problem, you will be given the % comp of the compound. 1. Base your calculation on 100 grams of compound. 2. Determine the # of moles of each element in 100 grams of the compound. (use molar mass) 3. Divide each value of moles by the smallest of the mole values. 4. Multiply each number by an integer to obtain all whole numbers.

Rhyme to help you remember 1. Percent to mass 2. Mass to mole 3. Divide by small 4. Multiple ‘til whole

Problem 1: Empirical Formula Calculation 1. Cinnamon contains cinnamaldehyde. A molecule of cinnamaldehyde contains 81.79% carbon, 6.10% hydrogen, and 12.11% oxygen. Determine the molecules empirical formula.

Empirical Formula Calculation Step 1: Percent to mass C: 81.79%  g H: 6.10%  6.10 g O: 12.11%  g

Empirical Formula Calculation Step 2: Mass to Mole C  81.79g x 1 mole/12g = 6.61 mol H  6.10g x 1 mole/1g = 6.05 mol O  12.11g x 1 mole/16g =0.757 mol

Empirical Formula Calculation Step 3: Divide by small C  6.61 mol/ mol = 9 H  6.05 mol/ mol = 8 O  mol/ mol = 1

Empirical Formula Calculation Step 4: Multiple ‘til whole C 9 H 8 O (in this case, we didn’t have to)

Problem 2: Empirical Formula Calculation 2. Methyl acetate is a solvent commonly used in some paints, inks, and adhesives. Determine the empirical formula for methyl acetate, which has the following chemical analysis: 48.64% carbon, 8.16 hydrogen, and 43.20% oxygen.

Empirical Formula Calculation Step 1: Percent to mass C: 48.64%  H: 8.16  O: 43.20% 

Empirical Formula Calculation Step 2: Mass to Mole C  48.64g x 1 mole/12g = H  8.16g x 1 mole/1g = O  43.20g x 1 mole/16g =

Empirical Formula Calculation Step 3: Divide by small C  mol / 2.70 mol = 1.5 H  8.10 mol / 2.70 mol = 3 O  2.70 mol / 2.70 mol = 1

Empirical Formula Calculation Step 4: Multiple ‘til whole C  1.5 x 2 = 3 H  3 x 2 = 6 O  1 x 2 = 2 C 3 H 6 O 2

Problem 3: Empirical Formula Determination 3. Adipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical formula of adipic acid?

Problem 3: Empirical Formula Determination 1. Percent to mass and 2. mass to mole

Empirical Formula Determination 3. Divide by small Carbon: Hydrogen: Oxygen:

Empirical Formula Determination 4. Multiple ‘til whole Carbon: 1.50Hydrogen: 2.50Oxygen: 1.00 x Empirical formula: C3H5O2C3H5O2

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Now let’s calculate Molecular Formulas from Percent Composition!

Molecular Formula Determination In these problems, either you will be given the empirical formula or you will be asked to calculate the empirical formula. You will also be given the molar mass of the compound.

1. Calculate the molar mass of the empirical formula. 2. Divide the given molar mass by the molar mass of the empirical formula. 3. Multiply the empirical formula by this number to get the molecular formula. Given molar mass = number of times Empirical molar mass Molecular Formula Determination

Finding the Molecular Formula 1. The empirical formula for adipic acid is C 3 H 5 O 2. The molar mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? 1. Find the molar mass of C 3 H 5 O 2 3(12.01 g) + 5(1.01) + 2(16.00) = g

Finding the Molecular Formula The empirical formula for adipic acid is C 3 H 5 O 2. The molar mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? 3(12.01 g) + 5(1.01) + 2(16.00) = g 2. Divide the molar mass by the mass given by the empirical formula

Finding the Molecular Formula The empirical formula for adipic acid is C 3 H 5 O 2. The molar mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? 3. Multiply the empirical formula by this number to get the molecular formula (C 3 H 5 O 2 ) x 2 =C 6 H 10 O 4

Pulling it all together 2. Naphthalene, commonly found in mothballs, is composed of 93.7% carbon and 6.3% hydrogen. The molar mass of naphthalene is 128 g/mole. Determine the empirical and molecular formulas for naphthalene.

Pulling it all together 3. Succinic acid is a substance produced by lichens. Chemical analysis indicates it is composed of 40.68% carbon, 5.08% hydrogen, and 54.24% oxygen and has a molar mass of g/mol. Determine the empirical and molecular formulas for succinic acid.

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