Calculating Grams to Moles (IGNORE)

Molar mass or molecular weight is a physical property that represents the mass of a substance divided by the amount of that substance. It serves as the bridge for the conversion between the grams and moles of a substance. 

How to Calculate Molar Mass

To calculate the molar mass of a compound we must determine the number of atoms for the elements within a given compound. The common units are typically in grams per mole (g/mol). 

For example, if we are asked to determine the molar mass of iron (II) nitrate, Fe(NO₃)₂, we can solve with steps shown below. 

STEP 1: Count the number of each element within the compound that contains 1 iron, 2 nitrogens and 6 oxygens. 

Chemical Formula (Counting Atoms)

Realize that the NO₃ group in parentheses has the “2” distributed to the nitrogen and oxygen. 

STEP 2: Find the atomic mass for each chemical element from the periodic table. 

Atomic Masses

STEP 3: Multiply the number of each element by their atomic masses taken from the periodic table.

Calculating masses  

STEP 4: Add up all the masses, obtained from multiplication, together to give the overall molar mass of the compound.

Molar Mass of Iron (II) Nitrate

Once you know the molar mass or molecular weight of a compound you can convert between grams and moles. 

How to convert from grams to moles

To convert from grams to moles we must divide the given gram amount by the molar mass of the compound. 

For example, if we are asked "how many moles are there in a 21.3 g Fe(NO3)2 sample?", this is how we should setup our problem:  

Grams to Moles Conversion

How to convert from moles to grams

Now what do we do when are given the moles of a compound and asked to determine its mass in grams?

For example, if we are asked, "how many grams are there in a 5.390 mole CH₂Cl₂ sample", this is how we should begin:  

STEP 1: Count the number of each element within the compound that contains 1 carbon, 2 hydrogens and 2 chlorines.

Chemical Formula (Counting atoms)

STEP 2: Find the atomic mass for each chemical element from the periodic table. 

Atomic Masses of Carbon, Hydrogen and Chlorine

STEP 3: Add up all the masses, obtained from multiplication, together to give the overall molar mass of the compound. 

Calculating molar mass (CH₂Cl₂)

STEP 3: Multiply the given mole amount by the molar mass of the compound. 

Moles to grams conversion

Beyond mass conversions

Mass conversions are a fundamental process in chemistry that will be used through numerous chapters. This concept will connect us to others such as molarity, stoichiometry, and solution chemistry where we deal with the concentration of homogeneous mixtures.