Download Empirical Formulas

Empirical
Formulas
Knowing exactly what it is
Percent Composition
 Breaks
down the amount of an element in
a compound by percentage
 Total mass of element / total mass of
compound
 Result will always be a decimal
 Multiply by 100 to get the final
percentage
Practice 1
What is the percent composition of the
elements in NaC2H3O2?
Practice 2
What is the percentage of Oxygen in
NH4NO3?
Practice 3
What is the percentage of Iron in Iron (III)
Carbonate?
Empirical formulas
 Formulas
are represented in smallest
whole number ratios
 Derived from percent composition of a
compound
 Remember 100g = 100 % for easier
calculations
Talk it Out…
1.
2.
3.
4.
Change percentage to grams assuming
there are 100g of the sample
Divide by the molar mass of each
element
Divide the moles of each element by the
lowest number of moles of all the
elements
Round to whole numbers to find ratios
UNLESS there is .5, then multiply each
result by 2 for whole number ratios
Problem 1
The percent composition of a compound
was found to be 63.5% Silver, 8.2% Nitrogen,
and 28.3% oxygen. Determine the
compound’s empirical formula.
Problem 2
A 170.00g sample of an unidentified
compound contains 29.84g of Sodium,
67.49g Chromium, and 72.67g Oxygen.
What is the compounds empirical formula?
Problem 3
A 60.00g sample of an tetraethyl lead, a
gasoline additive, contains 38.43g of Lead,
17.83g Carbon, and 3.74g Hydrogen. What
is the compounds empirical formula?
Molecular Formula
 You
must have the empirical formula first
 You must calculate the mass of the
empirical formula
 Don’t lose track of your numbers!
1.
2.
3.
4.
5.
Calculate empirical formula
Calculate the mass of the empirical
formula
Divide the given mass by the empirical
mass, this will give you an element
multiplier
Multiply your chemical quantities by the
multiplier
Rewrite molecular formula with new
chemical values
Problem 1
Determine the molecular formula of a
compound with an empirical formula of NH2
and a formula mass of 32.06 amu.
Problem 2
The empirical formula of a hydrocarbon is
found to be CH. Laboratory procedures
have found that the molar mass of the
compound is 78 g/mol. What is the formula
of this compound?
Problem 3
A compound containing 50.05% Sulfur and
49.95% Oxygen has a molar mass of 256.28
g/mol. Determine the empirical and
molecular formula of this compound.