let’s use those mole conversions!. empirical formula tells you the lowest ratio of atoms within a...

Empirical and Molecular Formulas

Let’s use those mole conversions!

Empirical formula tells you the lowest ratio of atoms within a molecule

Molecular formula tells you the actual ratio of atoms within a molecule

The empirical formula CAN equal the molecular formula, but it doesn’t have to.


Example:

A compound contains 39.99% carbon, 6.73% hydrogen, and 53.28% oxygen. The molar mass is 180.18 g/mol. What is the empirical and molecular formulas for this compound?


Step 1: ◦ Convert grams to moles

(If the problem gives you percents, assume that you have 100g of sample and use the percents as grams)


C:

H:

O:

Step 2: ◦ Divide all of the moles from Step 1 by the lowest

number of moles from Step 1.


C:

H:

O:

Lowest #

Step 2: ◦ Divide all of the moles from Step 1 by the lowest

number of moles from Step 1.


C: = 1

H:

O:

These numbers tell you howmany of each atom you have in the molecule.

Sometimes you may end up with non-whole numbers in Step 2. If this happens:

◦multiply all of the moles by 2 if any end in .5

◦multiply all of the moles by 3 if any end in .3


Step 3: ◦ Write the formula using the numbers found in

Step 2.


C: = 1

H:

O:

CH2O

This is the empirical formula:

If the problems asks for molecular formula, move on to Step 4:◦ Find the molar mass of the empirical formula and

divide the GIVEN molar mass by this number.


Molar mass of empirical formula:CH2O12.01g + 2*1.01g + 16.00g = 30.03 g/mol

GIVEN molar mass = 180.18 g/mol

180.18𝑔/𝑚𝑜𝑙30.03𝑔 /𝑚𝑜𝑙

=6

multiplier

Step 5: ◦ Multiply the number of atoms in the empirical

formula by the multiplier.


180.18𝑔/𝑚𝑜𝑙30.03𝑔 /𝑚𝑜𝑙

=6

multiplier

(CH2O) * 6 =

C6H12O6This is the molecular formula.

let’s use those mole conversions!. empirical formula tells you the lowest ratio of atoms within a...

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