WARM-UP

Write the formulas that go with the following names: Tricarbon Pentaoxide Osmium (III) Sulfide Magnesium Nitride Dinitrogen Tetraoxide

CHEMISTRY:EMPIRICAL AND MOLECULAR FORMULASUnit Five, Day Six

Kimrey

18 October 2012

EMPIRICAL FORMULA AND MOLECULAR FORMULA

Empirical formula is the simplest chemical formula Ex. CH4

Molecular formula is the chemical formula that can be reduced.

Molecular Formula- the formula for a compound in which the subscripts give the actual number of each element in the formulas it truly exists.

Ex. C2H8

NOTICE TWO THINGS:1. THE MOLECULAR FORMULA AND THE

EMPIRICAL FORMULA CAN BE IDENTICAL.2. YOU SCALE UP FROM THE EMPIRICAL

FORMULA TO THE MOLECULAR FORMULA BY A WHOLE NUMBER FACTOR.

Molecular Formula Empirical Formula

H2O

CH3COOH

CH2O

C6H12O6

CALCULATING THE EMPIRICAL FORMULA FROM THE PERCENT COMPOSITION

Steps1. Assume the percent = grams. 2. Convert each amount of grams to moles. 3. Divide each number of moles by the

smallest number of moles. 4. If all the numbers are not whole numbers,

multiple everything by a number that will get you a whole number. (This may take a few tries!)

5. The whole numbers become subscripts in the formula.

Percent to mass Mass to mole Divide by small Multiply ‘til whole

EXAMPLE

A compound is 32.38% sodium, 22.65% sulfur, and 44.99% oxygen. What is the empirical formula?

EXAMPLE 2

A compound is 40.0% Carbon; 6.71% Hydrogen; 53.29% Oxygen. What is its empirical formula?

PRACTICE

89.94% C; the rest is H 

56.34% P; 43.66% O  

43.64% P; 56.36% O  

40.9% C; 4.58% H; 54.5% O  

NOW, FINDING THE MOLECULAR FORMULA

1) Find the empirical formula2) Calculate the molar mass (formula mass)3) Divide the given mass of the substance by

the molar mass4) Multiply each subscript of the empirical

formula by the answer to #3

EXAMPLE

You have 56.106 grams of a substance with an empirical formula of CH2. What is the molecular formula?

CuSO4 ● 5H2O

MgSO4 ● 7H2O   SnCl2 ● 2H2O

Na2CO3 ● 10H2O