How to interpret and use chemical formula to go from molecules of one substance to grams of another

To use a chemical formula to go from molecules to atoms or grams, we must first learn how to interpret the chemical formula in terms molecules and atoms. Once we do, we can then proceed to write ratio identities (conversion factors). We can write this ratio identities because the law of constant composition says that atoms combine in fixed mass ratio. Now, let’s further explain this using the chemical formula for methane: CH₄

As you can tell, CH₄ is just a single molecule of methane. As a result, we can interpret CH₄ in terms of atoms by specifying the number of each type of atom in it. For instance, we can say that; 1 molecule of CH₄ contains 1 atom of carbon, and also contains 4 atoms of hydrogen. If we replace the word contains with an equal sign, then we can write the following expression:

1 molecule of CH₄ = 1 atom of C = 4 atoms of H

From the above expression, we can write the following molecule to atom ratios or atom to atom ratios:

1 molecule of CH₄ / 1 atom of C or 1 atom of C / 1 molecule of CH₄

1 molecule of CH₄ / 4 atoms of H or 4 of atoms of H / 1 molecule of CH₄

1 atom of C / 4 atoms of H or 4 atoms of H / 1 atom of C

Notice that the forward slash (/) denotes division.

Here is a much clearer way of representing the above ratios:

So, as you may have noticed, these are the ratio identities we will use to help convert between molecules and atoms or between moles and atoms and vice versa. Now, let’s test our understanding by solving the following problem:

Question

What is the mass of oxygen (O) atoms present in 2.02 x10²³ molecules of CO₂?

Strategy

We will convert from molecules of CO₂ to atoms of O and from atoms of O to moles of O and from moles of O to grams of O.

To go from molecule of CO₂ to atoms of O, we will use the the conversion factor: 2O atoms/1 CO₂ molecule ( ratio is from CO₂ chemical formula).

To go from atoms of O to moles of O, we will use Avogadro’s number and mole as conversion factor. That is: 1 mol O/6.02 x10²³ atoms of O.

To go from atoms of O to grams of O, we will use the molar mass of O.

Here is the complete setup:

To learn how to interpret and use chemical formula to go from moles to moles, or atoms, or grams, click here.