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Note taking Guide: Percent Composition, Empirical and Molecular Formulas

Percent Composition

• A ______________________________ 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 _____________.

• 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 Fe2O3 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 calculating percent composition

1. Calculate the _____________________ of an element

2. Calculate the _____________________ of the compound.

3. _____________________ the mass of the element by the molar mass of the substance.

4. _____________________ to make it a percentage.

Percent = mass of element in 1 mole x 100

Comp molar mass of compound

Let’s try some together:

1. What is the percent composition by mass of carbon in carbon dioxide (CO2)?

2. What is the percent composition of both hydrogen and oxygen in water?

STOP: complete Percent Composition Worksheet and POGIL Activity

Determining formulas from percent composition

• _____________________: the true number of atoms of each element in the formula of a compound. Example: C6H6 and (CH)6

• _____________________: the lowest whole number ratio of atoms in a compound. Example: CH

• Formulas for ionic compounds are _____________________ empirical (lowest whole number ratio).

• Formulas for covalent (molecular) compounds _____________________ be empirical (lowest whole number ratio).

Determining Empirical Formulas

In the problem, you will be given the % composition of the compound.

1. Base your calculation on _____________________ of compound.

2. Determine the _____________________ of each element in 100 grams of the compound. (use molar mass)

3. _____________________ each value of moles by the _____________________of the mole values.

4. _____________________ each number by an integer to obtain all _____________________.

Rhyme to help you remember

1.

2.

3.

4.

Let’s try some Empirical Formula problems

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

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.

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

STOP : Work with a partner to complete determining empirical formulas

Determining Molecular or True Formulas

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 _____________________ of the empirical formula.

2. _____________________ the given molar mass by the molar mass of the empirical formula.

3. _____________________ the empirical formula by this number to get the molecular formula.

Given molar mass = number to multiple the empirical formula by

Empirical molar mass

Let’s try a Molecular Formula problem

1. The empirical formula for adipic acid is C3H5O2. The molar mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?

Let’s pull 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.

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 118.1 g/mol. Determine the empirical and molecular formulas for succinic acid.

STOP : Work with a partner to complete determining empirical formulas

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