Molecular Formula from %Composition & Molecular Mass
This will guide you through determining the molecular formula of a compound just based on the percent composition and molecular mass.
Molecular Formula: The actual formula for a molecule.
Problem: A compound is 75.46% carbon, 4.43% hydrogen, and 20.10% oxygen by mass. It has a molecular weight of 318.31 g/mol. What is the molecular formula for this compound?
- Find the empirical formula.
- Get the mass of each element by assuming a certain overall mass for the sample (100 g is a good mass to assume when working with percentages).
- Convert the mass of each element to moles.
- Find the ratio of the moles of each element.
- Use the mole ratio to write the empirical fomula.
- Find the mass of the empirical unit.
- Figure out how many empirical units are in a molecular unit.
- Write the molecular formula.