Formulas and Formulas and Percent Percent

CompositionCompositionFinding the Mystery Finding the Mystery

FormulasFormulas

Using Analytical DataUsing Analytical Data

Scientist synthesize new compounds Scientist synthesize new compounds for many uses. for many uses.

Once they make a new product, they Once they make a new product, they must check its identity. must check its identity.

One way to carry out a chemical One way to carry out a chemical analysis is by providing a analysis is by providing a percent percent composition.composition.

Example in HistoryExample in History

In 1962, scientists made a new compound In 1962, scientists made a new compound from xenon and fluorine. from xenon and fluorine.

Before 1962, scientist believed xenon did not Before 1962, scientist believed xenon did not form compounds. form compounds.

The scientists did a percent composition of The scientists did a percent composition of the compound and found it was 63.3% Xe and the compound and found it was 63.3% Xe and 36.7 % F, which is the same as that for a 36.7 % F, which is the same as that for a formula XeFformula XeF44. .

Percent Composition helps identify a Percent Composition helps identify a substance by comparing ratio of masses substance by comparing ratio of masses (empirical formula).(empirical formula).

Determining % Determining % CompositionComposition

% Composition % Composition = = Mass of part of sampleMass of part of sample

Mass of whole sampleMass of whole sample

*% composition remains consistent regardless of *% composition remains consistent regardless of the size of the samplethe size of the sample

X 100

Example % Comp Example % Comp ProblemProblem

Ex. Find the % composition of Ex. Find the % composition of Ca, O and H in a Ca(OH)Ca, O and H in a Ca(OH)22 sample. sample.

Based on a 20.18 g sample of Based on a 20.18 g sample of Ca(OH)Ca(OH)22, how many grams did , how many grams did the individual elements the individual elements contribute?contribute?

Determining Empirical Determining Empirical FormulasFormulas

Data for a percent composition allow Data for a percent composition allow you to calculate the you to calculate the simplest ratio simplest ratio among the atoms found in a compound.among the atoms found in a compound.

Empirical Formula is the simplest Empirical Formula is the simplest ratio of atoms in a compoundratio of atoms in a compound

Ammonium Nitrite formula is NHAmmonium Nitrite formula is NH44NONO22

Ammonium Nitrite empirical Formula Ammonium Nitrite empirical Formula is NHis NH22OO

Percent Composition To Percent Composition To Empirical FormulaEmpirical Formula

*Step 1 – change % to grams, assuming a 100 g *Step 1 – change % to grams, assuming a 100 g samplesample

*Step 2 – convert grams to moles*Step 2 – convert grams to moles *Step 3 – divide all mole values by the smallest *Step 3 – divide all mole values by the smallest

mole value to try and get all values into simple mole value to try and get all values into simple whole numberswhole numbers

*Step 4 – if mole values are not whole numbers *Step 4 – if mole values are not whole numbers after step 3, multiply all mole values by the same after step 3, multiply all mole values by the same smallest whole number to achieve this smallest whole number to achieve this 

*Step 5 – write the empirical formula of the *Step 5 – write the empirical formula of the compound compound

  

Example ProblemsExample Problems

A compound is found to have 79.8% A compound is found to have 79.8% carbon and 20.2% hydrogen by carbon and 20.2% hydrogen by mass. What is the empirical formula mass. What is the empirical formula of this compound?of this compound?

A compound is composed of 58.8% A compound is composed of 58.8% C, and 9.8% H. What is the C, and 9.8% H. What is the empirical formula of this compound?empirical formula of this compound?

More Practice ProblemsMore Practice Problems

60.0% C, 13.4% H, 26.6% O. Find 60.0% C, 13.4% H, 26.6% O. Find the empirical formula.the empirical formula.

A dead battery is found to contain a A dead battery is found to contain a compound of 69.9% Mn and 30.4% compound of 69.9% Mn and 30.4% O. What is the empirical formula?O. What is the empirical formula?

Magnetic iron oxide is 72.4% iron Magnetic iron oxide is 72.4% iron and 27.6% oxygen. What is the and 27.6% oxygen. What is the empirical formula?empirical formula?

Empirical formulas not Empirical formulas not molecular formulasmolecular formulas

In determining empirical formulas, In determining empirical formulas, the simplest ratio of atoms was the simplest ratio of atoms was determined. This is not necessarily determined. This is not necessarily the exact number of atoms in the the exact number of atoms in the compound which is known as the compound which is known as the molecular formula. molecular formula.

Additional information is needed to Additional information is needed to determine molecular formulas.determine molecular formulas.

Molecular FormulasMolecular Formulas

Molecular Formulas are made of Molecular Formulas are made of single molecules. single molecules.

For many compound the For many compound the molecular molecular formula is a simple whole number formula is a simple whole number ratio of the empirical formularatio of the empirical formula

The molar mass of a molecular The molar mass of a molecular formula is equal to the molar mass of formula is equal to the molar mass of a empirical formula multiplied by “n” a empirical formula multiplied by “n” (a whole (a whole nnumber)umber)

ProcessProcess

If you divide the experimental molar If you divide the experimental molar mass by the molar mass of the mass by the molar mass of the empirical formula, you can figure out empirical formula, you can figure out the value of the value of nn needed to scale the needed to scale the empirical formula up to give the empirical formula up to give the molecular formula.molecular formula.

n(emperical formula) = molecular n(emperical formula) = molecular formulaformula

Example ProblemExample Problem

The empirical formula of a The empirical formula of a compound is CHcompound is CH33N. The mass of the N. The mass of the molecular formula is 60.12 g/mol. molecular formula is 60.12 g/mol. What is the molecular formula of this What is the molecular formula of this compound?compound?

Steps To Find The Steps To Find The Molecular Formula From Molecular Formula From The Empirical FormulaThe Empirical Formula

1.1. Find molar mass of the empirical formula Find molar mass of the empirical formula

2.2. Divide the molecular mass given in the Divide the molecular mass given in the problem by the empirical formula mass to problem by the empirical formula mass to get the multiple of how many times larger get the multiple of how many times larger the molecular mass is compared to the the molecular mass is compared to the empirical mass (n).empirical mass (n).

3.3. Use the multiple you found in step 2 to write Use the multiple you found in step 2 to write your molecular formula. This must mean your molecular formula. This must mean that the # of atoms of the empirical formula that the # of atoms of the empirical formula must be multiplied by this multiplemust be multiplied by this multiplen(empirical formula)n(empirical formula)

More Example ProblemsMore Example Problems

Molar mass of 78 g/mol and empirical Molar mass of 78 g/mol and empirical formula of CH, what is its molecular formula of CH, what is its molecular formula?formula?

A brown gas on the cabinet to your A brown gas on the cabinet to your

right has the empirical formula of NOright has the empirical formula of NO22. . Its experimental molar mass is 46g/mol. Its experimental molar mass is 46g/mol. What is its molecular formula?What is its molecular formula?

The lichens presents on the bark of trees The lichens presents on the bark of trees contain a substance called succinic acid. contain a substance called succinic acid. The organic acid is also present in fungi, The organic acid is also present in fungi, which is used to make dyes and pefumes. which is used to make dyes and pefumes.

Percent composition is 40.68% C, 5.08% Percent composition is 40.68% C, 5.08% H, 54.24 % O. H, 54.24 % O.

Its molecular mass is 118.8 g/molIts molecular mass is 118.8 g/mol What is its empirical and molecular What is its empirical and molecular

formulas?formulas?