Converting grams to moles is a problem that occurs when a measured amount of mass is known but the ratio for balanced reactions is needed. These two example problems show the best way to convert grams to moles and moles to grams of a molecule.
Grams to Moles Example Problem
Question: How many moles are in 110.0 grams of CO2.
First, we need to know how many grams are in 1 mole of CO2. This is done by finding the molecular mass of CO2. CO2 has 1 carbon atom and 2 oxygen atoms. Looking at the periodic table, we can find the atomic mass of both carbon and oxygen.
atomic mass of carbon = 12.0 g/mol
atomic mass of oxygen = 16.0 g/mol
molecular mass of CO2 = 1⋅(atomic mass of C) + 2⋅(atomic mass of O)
molecular mass of CO2 = 1⋅(12.0 g/mol) + 2⋅(16.0 g/mol)
molecular mass of CO2 = 12.0 g/mol + 32.0 g/mol
molecular mass of CO2 = 44.0 g/mol
This means there are 44.0 grams in one mole of CO2. Use this value as a conversion factor to find the number of moles of CO2 in our problem.
This setup of the conversion factor cancels out the grams, leaving the mole unit we need.
# moles CO2 = 2.5 moles
Answer: There are 2.5 moles of CO2 in 110.0 grams of CO2.
Moles to Grams Example Problem
Question: What is the mass in grams of 4.80 moles of hydrogen peroxide (H2O2)?
Just like the first example, we need to know the molecular mass of hydrogen peroxide. H2O2 has two hydrogen atoms and two oxygen atoms.
atomic mass of H = 1.01 g/mol
atomic mass of O = 16.00 g/mol
molecular mass of H2O2 = 2⋅(atomic mass of H) + 2⋅(atomic mass of O)
molecular mass of H2O2 = 2⋅(1.01 g/mol) + 2⋅(16.00 g/mol)
molecular mass of H2O2 = 2.02 g/mol + 32.00 g/mol
molecular mass of H2O2 = 34.02 g/mol
Now we know 1 mole of H2O2 has a mass of 34.02 grams. This value gives us our conversion factor to find the mass of the H2O2.
mass of H2O2 = 163.30 grams
4.80 moles of hydrogen peroxide has a mass of 163.30 grams.
To convert grams to moles or moles to grams:
- Find the molecular mass of the molecule.
- Use this value as a conversion factor.
- Multiply the amount by the conversion factor, making sure the undesired value is canceled out.
The most common place an error is likely to occur is setting up the conversion factor incorrectly. Check to make sure the correct unit cancels out.