Empirical and Molecular Formulas

Percent Composition

I can determine the percent composition for

each element in a compound or sample.

Note Expectations:

Cell phones and electronics are not in use.

You are taking the notes. You are helping the people at your

table to answer the questions. You are prepared to answer the

questions.

What is a percent? On the mole test, you earned 25 out

of 33. What percent did you get on the test?

How do you calculate a percent?

What does the formula C6H12O6

tell us? How do you find the mass of a

compound?

The formula for a

compound indicates the number and kinds of atoms in a representative particle of the compound.

The mass of a compound is made up of smaller masses of different elements.

Formulas

What is the percent of

hydrogen and the percent of oxygen in H2O? Show how you think you would solve this problem.

Work as a Table

The percent composition of a

compound tells you the percent mass made up by each element in the compound.

The percent composition can be determined by taking the mass of individual elements and dividing by the total mass.

Percent Composition

Then, you multiply by 100

to get the percent composition.

Percent Composition

What is the percent

composition of H and O in H2O2?

Example

What is the percent

composition of N and O if 22.6g of N and 54.4 g of O are present in a 77.0 gram sample?

Once you have found all of

the percents, they should add up to 100%, or close to it based on rounding.

Check your work!

1. What is a percent?2. How do you calculate a percent? You can answer this with an example.3. What does a formula tell you?4. What does percent composition tell you?5. What is the total mass of a compound made of?

Review Questions

Empirical Formulas

I can define empirical formula. I can determine the empirical

formula if given the elements in the formula.

Note Expectations:

Cell phones and electronics are not in use.

You are taking the notes. You are helping the people at your

table to answer the questions. You are prepared to answer the

questions.

What is a ratio? What is the ratio of

hydrogen to carbon in C6H12O6 ?

A formula that gives the

simplest whole number ratio of the atoms of the elements is called an empirical formula.

Empirical Formula

What would the simplest

ratio of C6H12O6 be?

The molecular formula for

peroxide is H2O2. The empirical formula for

peroxide is HO. The empirical formula simply

tells you the ratio of atoms, in this case 1:1.

Formulas

To determine the empirical

formula based on % composition, you assume 100g.

This will make the math easier. Next, you convert each of the

masses to moles using the molar mass of the element.

Procedure

How do you convert mass

to moles? Where does the mass of an

element come from?

Review Question

You divide each element by the

smallest number of moles. Round your answers to the nearest

whole number. These then become the subscripts for

each element in the formula. This is your ratio for the empirical

formula.

Procedure

A compound has a percent

composition of 80% C and 20% H, what is its empirical formula?

Example

If you are given grams

instead of percents, you can start by converting to moles.

A compound was found to have

13.5g Ca, 10.8g O, and 0.675g H, what is its empirical formula?

Example

1. What is a ratio?2. What is an empirical formula?3. Summarize the steps required to calculate an

empirical formula.

Review Questions

Calculating Molecular Formulas

I can define molecular formula. I can determine the molecular formula if given

the proper information.

Note Expectations:

Cell phones and electronics are not in use.

You are taking the notes. You are helping the people at your

table to answer the questions. You are prepared to answer the

questions.

Is the empirical formula the

actual formula? How do you think it compares?

Could the empirical formula be the actual formula? Why or why not?

The molecular formula can

be the same as the empirical formula.

However, most of the time it is a whole number multiple of the empirical formula.

Molecular Formulas

You can determine the

molecular formula of a compound if you know its molar mass.

First, you find the mass of the empirical formula.

Procedure

This is called the empirical

formula mass or efm. Then you take the known

molar mass and divide by the efm.

Procedure

This gives you the number of

empirical formula units in one molecule of the compound.

Take the number you got and round to the nearest whole number.

Procedure

Then, multiply all of the

subscripts in the formula by that number.

This will give you the molecular formula.

Procedure

Calculate the molecular formula of

the compound whose molar mass is 60.0g and empirical formula is CH4N.

Example

What is a molecular formula? How is the molecular formula

related to the empirical formula?

Summarize the steps to calculate the molecular formula.

Review Questions