Introduction to Compounds

We know all about the structure of atoms and how electrons are arranged. Now, we will begin looking at how we can rearrange electrons to form chemical bonds. Chemical bonds hold atoms together forming compounds.

If you would like a refresher on how atoms differ from compounds, please see the video on "Atoms vs. Molecules."

In this unit we will expand on the concept of bonding and learn about some finer details of molecular structure. Like atoms, molecules can be counted by mass. The molar mass of a compound is the mass of 1 mole of a substance. It can be calculated by added the molar mass of each individual atom present.

Let's use aspirin as an example. The common analgesic has a molecular formula of $C_9H_8O_4$. The molecular formula tells you exactly how many and what type of atoms are present in a molecule. To calculate the molar mass, we would carry out the following calculation:

$\text{molar mass} = (9\text{ mol C atoms})(12.011\text{ g/mol C})+(8 \text{ mol H atoms})(1.008 \text{ g/mol H})+(4 \text{ mol O atoms})(15.999 \text{ g/mol O})$ $\text{molar mass} = 180.157 \text{ g/mol}$

The mass of a substance can be converted to moles by dividing the mass by the molar mass. Similarly, the number of moles of a substance can be converted to grams by miultiplying the moles by the molar mass.

The number of moles of a substance can be multiplied by Avogadro's number ($N_A$) to determine how many molecules are present. Similarly, you can divide the number of molecules by $N_A$ to determine the number of moles that are present.

Example: How many aspirin molecules are in a 325 mg dose of aspirin?

$325\text{ mg}(\frac{1\text{ g}}{1000\text{ mg}})(\frac{1\text{ mol}}{180.157\text{ g}})(\frac{6.022\times10^{23}\text{ molecules}}{1\text{ mol}}) = 1.09\times10^{21} \text{molecules}$

An extension of this concept is calculating the number of atoms of a particular type in a compound.

Example: How many carbon atoms are in a 325 mg dose of aspirin?

The first 3 steps of the calculation are the same as above. The 4th step is to use the mole ratio. The mole ratio tells how many atoms per molecule there are and is determined from the molecular formula. Aspirin has 9 carbon atoms per molecule and can be written as $\frac{9\text{ C atoms}}{1\text{ molecule}}$ (or the reciprocal).

$325\text{ mg}(\frac{1\text{ g}}{1000\text{ mg}})(\frac{1\text{ mol}}{180.157\text{ g}})(\frac{6.022\times10^{23}\text{ molecules}}{1\text{ mol}})(\frac{9\text{ C atoms}}{1\text{ molecule}}) = 9.78\times10^{21} \text{C atoms}$