Chapter 11 Notes IVEmpirical/Molecular Empirical/Molecular FormulaFormula

Empirical FormulaEmpirical Formula

The empirical formula gives The empirical formula gives you the lowest, whole you the lowest, whole number ratio of elements in number ratio of elements in the compound.the compound.

The empirical formula may or The empirical formula may or may not be the same as the may not be the same as the molecular formula.molecular formula.

For example:For example:

For carbon dioxide, the For carbon dioxide, the molecular formula is COmolecular formula is CO22, and , and the empirical formula is COthe empirical formula is CO22. . One carbon and two oxygens One carbon and two oxygens are the lowest ratio of atoms.are the lowest ratio of atoms.

The molecular formula for The molecular formula for dinitrogen tetrahydride is Ndinitrogen tetrahydride is N22HH44, , but the empirical formula is but the empirical formula is NHNH22..

What is the empirical What is the empirical formula for:formula for:

CC66HH1212OO66

CC66HH1212OO22

NN22HH22

CHCH44

Obj. 10-11…Empirical FormulasObj. 10-11…Empirical Formulas

• empirical formulasempirical formulas are the lowest whole # ratio of a are the lowest whole # ratio of acompound.compound.

• i.e. the empirical formula for glucose (Ci.e. the empirical formula for glucose (C66HH1212OO66) would be…) would be…

CHCH22O O

• to calculate from % composition…to calculate from % composition…

1. 1. ÷÷ given % by atomic mass given % by atomic mass (to find moles)(to find moles)

2.2. ÷÷ each mole in step 1 by lowest mole value each mole in step 1 by lowest mole value (to find ratio) (to find ratio)

3. 3. values from step 2 = subscripts values from step 2 = subscripts

~~ subscripts subscripts MUSTMUST be a whole # be a whole # (multiply if they are not!)(multiply if they are not!)

To Find the Empirical Formula To Find the Empirical Formula from % Composition:from % Composition:

If given the percentages, If given the percentages, assume there are 100.0 grams assume there are 100.0 grams of the compound.of the compound.

Convert the grams of each Convert the grams of each element to moles.element to moles.

Divide by the smaller amount of Divide by the smaller amount of moles, then manipulate the moles, then manipulate the ratio so that all numbers are ratio so that all numbers are whole.whole.

Obj. 10-11 cont…Obj. 10-11 cont…• practice…practice…

1.1. What is the empirical formula of a compound thatWhat is the empirical formula of a compound that

contains 36.5% sodium, 25.4% sulfur and 38.1% oxygen?contains 36.5% sodium, 25.4% sulfur and 38.1% oxygen?

NaNa == 36.5%36.5%

2323== 1.59 moles1.59 moles

SS == 25.4%25.4%

3232== 0.794 moles0.794 moles

OO == 38.1%38.1%

1616== 2.38 moles2.38 moles

******

0.7940.794== 22

0.7940.794 == 11

0.7940.794== 33

subscriptssubscriptsNa SONa SO22 33

Obj. 10-11 cont…Obj. 10-11 cont…• more practice…more practice…

2.2. What is the empirical formula of a compound thatWhat is the empirical formula of a compound that

contains 72.4% iron (Fe) and 27.6% oxygen?contains 72.4% iron (Fe) and 27.6% oxygen?

FeFe == 72.4%72.4%

5656== 1.293 moles1.293 moles

OO == 27.6%27.6%

1616== 1.725 moles1.725 moles

****** 1.2931.293

== 11

1.2931.293== 1.331.33

MUST be a whole #!!!MUST be a whole #!!!

x 3 x 3 == 44

x 3 x 3 == 33

Fe OFe O33 44

Two Examples:Two Examples:

What is the empirical formula What is the empirical formula of a compound that is 27.3% of a compound that is 27.3% carbon and 72.7% oxygen?carbon and 72.7% oxygen?

What is the empirical formula What is the empirical formula of a compound that is 25.9% of a compound that is 25.9% nitrogen and 74.1% oxygen?nitrogen and 74.1% oxygen?

More to try…More to try…

Calculate the empirical formula Calculate the empirical formula of a compound that is 94.1% of a compound that is 94.1% oxygen, 5.9% hydrogen.oxygen, 5.9% hydrogen.

Calculate the empirical formula Calculate the empirical formula of a compound that is 79.8% of a compound that is 79.8% carbon, 20.2% hydrogen.carbon, 20.2% hydrogen.

From there, find it’s molecular From there, find it’s molecular formula:formula:

If given the molar mass (how many If given the molar mass (how many g/mol of the compound) then you g/mol of the compound) then you can calculate the molecular can calculate the molecular formula from the empirical formula from the empirical formula.formula.

Take the compound’s empirical Take the compound’s empirical formula mass and compare to the formula mass and compare to the molecular mass. The molecular molecular mass. The molecular mass will be a multiple of the mass will be a multiple of the empirical formula’s mass.empirical formula’s mass.

Example:Example:

If the molecular mass of the If the molecular mass of the first example (Nfirst example (N22OO55) problem ) problem is 216 g/mol, then what is the is 216 g/mol, then what is the molecular formula for the molecular formula for the compound?compound?

Try these:Try these:

What is the molecular formula What is the molecular formula of a compound whose molar of a compound whose molar mass is 60.0 g and whose mass is 60.0 g and whose empirical formula is CHempirical formula is CH44N?N?

What is the molecular formula What is the molecular formula for a compound whose molar for a compound whose molar mass is 78 g and whose mass is 78 g and whose empirical formula is CH?empirical formula is CH?

Obj. 10-11 cont…Obj. 10-11 cont…• calculate hydrated compounds (bonded to water) same way…calculate hydrated compounds (bonded to water) same way…

~~ use ‘use ‘’ to separate water from crystal’ to separate water from crystal~~ since water is a compound, you are finding the since water is a compound, you are finding the coefficientcoefficient……

NOT the subscript!NOT the subscript!• practice…practice…

What is the formula of a compound that contains 18.28% Ca,What is the formula of a compound that contains 18.28% Ca, 32.36% Cl and 49.36% H32.36% Cl and 49.36% H22O?O?

CaCa == 18.28%18.28%

4040== 0.457 moles0.457 moles

ClCl == 32.36%32.36%

3535== 0.925 moles0.925 moles

HH22OO == 49.36%49.36%

1818== 2.742 moles2.742 moles

****** 0.4570.457

== 11

0.4570.457

0.4570.457== 66

== 22 CaCl CaCl H H22OO2266