Jan 9th - Mole, mole, molecular Formulas!

To finish off our unit about moles, we learned about molecular formulas. To figure out the molecular formula of a substance, we need to know two pieces of information. This includes the empirical formula and molar mass of the substance. Once we have these, the molecular formula can be easily determined.

Before we begin to solve questions, let us describe why a molecular formula is important. Previously, we learned that the empirical formula is the lowest ratio of atoms in a compound. Unfortunately, this does not contain much information. For example, let us consider NO. Knowing just the empirical formula, the actual molecular formula can have infinite possibilities, such as NO, N₂O₂, N₄O₄, N₁₆O₁₆­, etc.

This is why molecular formulas are important. Firstly, these formulas tell us how many of each atom is in a compound. In addition, it narrows down the possibilities of different arrangement of the atoms, giving us the shape of the substance.

Empirical	Molecular
(formula)	(formula)
(molar mass)	(molar mass)

This chart will help us accomplish our task. It requires us to fill in the boxes with the appropriate information.

Now that we are equipped with the knowledge about molecular formulas, we can take on some problems. Let’s begin with an easy one.

1) A substance is made up of palladium and hydrogen and has a mass of 43.36g. It contains 42.56g of palladium. The molar mass of the compound is 216.8 g/mol.

First, we begin by determining the empirical formula. Let’s make our chart.

Atom	Mass	Molar Mass	Moles	Mole/ Smallest Mole	Ratio
Pa H	42.56g 0.80g	106.4g/mol 1.0 g/mol	0.40mol 0.80 mol	1 2	1 2

We have determined our empirical formula to be H₂Pa. *The order of writing the elements, at the moment, does not matter. However, sometimes there is obviously a correct way.

We will now form another chart, which will be our aid to determining molecular formulas.

Empirical	Molecular
H2Pa	H4Pa2
108.4g/mol	218.6g/mol

Our empirical formula gives us only half of our needed mass. In order to achieve this, we have to simply multiplying each atom in our compound by two. It’s as simple as that. Therefore, our molecular formula is: H₄Pa₂.

Let’s quickly try another one.

A substance in our bodies is made up of 95.992 grams carbon, 10.985 grams hydrogen, 47.996 grams oxygen, and 14.027 grams nitrogen and has a molar mass of 338g/mol. Determine the molecular formula.

Using one mole of the substance, we receive:

Atom	Mass	Molar Mass	Moles	Mole/ Smallest Mole	Ratio
C H O N	95.992g 10.985g 47.996g 14.027g	12.0g/mol 1.0g/mol 16.0g/mol 14.0g/mol	8 11 3 1	8 11 3 1	8 11 3 1

Our empirical formula is: C₈H₁₁NO₃.

Empirical	Molecular
C8H11NO3	C16H22N2O6
169g/mol	338g/mol

Our molecular formula is: C₁₆H₂₂N₂O₆! That’s it!

Posted by Andrew.