DETERMINING CHEMICAL FORMULAS

What if you discovered the cure for cancer?

Any new chemical that is discovered that is a potential health care treatment must have its molecular formula submitted to the Ministry of Health.

How do you determine the molecular formula of some new compound that neither your nor anyone may have never seen before?

It’s actually not that hard…all you need is a device called a mass spectrometer and some chemistry fundamentals – some of which are hundreds of years old!

The Mass Spectrometer

A mass spectrometer is a lab instrument that measures the molar mass of a compound.

The machine can also give you a reading of each element in the compound based on mass and percentage.

Look at this reading from the Cassini Spacecraft as it flew by one of Saturn’s moons – Enceladus.

Law of Constant Composition

A compound contains elements in certain fixed proportions (ratios) and in no other combinations.

This law is important in formula determination because it basically says that each compound has only one formula…if you find something that matches a known formula then that is what you have found – no arguments.

Water will always be H2O…Find that and you have found water.

Empirical vs. Molecular Before you can determine the molecular formula of a

compound you must first determine the empirical formula. An empirical formula is the lowest whole number ratio of

elements in a compound. Think of empirical as a formula in lowest terms.

The molecular formula of a compound is the actual number of atoms in one molecule of a compound.

Think of molecular formulas as seeing the compound out on the street.

Example: Glucose C6H12O6 (molecular formula)

CH2O (empirical formula)

Determining Empirical Formulas

In order to determine the empirical formula for a compound you need the percent composition for it…

The mass spectrometer will give you this…or the question will tell you!

Let’s try an example of a problem in which we must determine the percent composition.

Sample Problem 1: The substance that gives sour milk that lovely taste is lactic acid. It consists of 40.00% Carbon and 6.71% Hydrogen with the remainder being oxygen. Determine its empirical formula…Or Else!!!

C = 40.00g ÷ 12.01g/mol = 3.33mol C = 3.33mol ÷ 3.33mol = 1

H = 6.71g ÷ 1.01g/mol = 6.64mol H = 6.64mol ÷ 3.33mol = 2

O = 53.29g ÷ 16.00g/mol = 3.33mol O = 3.33mol ÷ 3.33mol = 1

Therefore the empirical formula for lactic acid is CH2O.

Steps to Solve Assume 100g

(% g) g mol

(÷ by g/mol) Divide by

lowest mol

Empirical Is Not Enough! If you look at this slideshow carefully –

you will have noticed both glucose (C6H12O6) and lactic acid (C3H6O3) have the same empirical formula – but they are very different molecules it terms of properties and function within the human body!

Many molecules can have the same empirical formulas so it is absolutely necessary to be able to determine the molecular formula for any compound.

In order to determine the molecular formula we need two pieces of information: Empirical Formula (or a way to get it…like percent

composition…a la the mass spectrometer) Molar mass of the compound (The almighty mass

spectrometer will give us this too!)

Behold!...The Mass Spec!

Sample Problem 2: Determine the molecular formula of a compound containing 85.7% carbon and 14.3% hydrogen by mass. The molar mass of the compound is 84g/mol. Solve…Or Else!!!

We do not have the empirical formula but we do have a way to get it. (by using the Percent Composition)!

C = 85.7g ÷ 12.01g/mol = 7.136mol ÷ 7.136mol = 1

H = 14.3g ÷ 1.01g/mol = 14.16mol ÷ 7.136mol = 2

Therefore the empirical formula is CH2.

Molar Mass = 84g/mol = 6Empirical Formula Mass 14.03g/mol

This means that the molecular formula is 6 times larger than the empirical formula so we need to multiply the empirical formula by 6 to get the molecular formula.

6 x CH2 = C6H12

The Molecular formula is C6H12.

Sample Problem 3: Determine the molecular formula of a compound with the empirical formula P2O3 and a molar mass of 220.0g/mol.

We are given the empirical formula outright so you go directly to the second step of the solving process.

Molar Mass = 220.0g/mol = 2

Empirical Formula Mass 109.94g/mol

This means that the molecular formula is 2 times larger than the empirical formula so we need to multiply the empirical formula by 2 to get the molecular formula.

2 x P2O3 = P4O6

The molecular formula is P4O6.

That’s All I Got!...

“I know it’s a lot to think about…

So I will leave you to it.”