Molecular Formula 2
Molecular Formula: The actual formula for a molecule.
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).
(.7546) (100 g) = 75.46 g C
(.0443) (100 g) = 4.43 g H
(.2010) (100 g) = 20.10 g O
- Convert the mass of each element to moles.
(75.46 g C) (1 mol/ 12.00 g C) = 6.289 mol C
(4.43 g H) (1 mol/ 1.008 g H) = 4.39 mol H
(20.10 g O) (1 mol/ 16.00 g O) = 1.256 mol O
- Find the ratio of the moles of each element.
(1.256 mol O)/ (1.256) = 1 mol O
(6.289 mol C)/ (1.256) = 5.007 mol C
(4.39 mol H)/ (1.256) = 3.50 mol H
- Use the mole ratio to write the empirical fomula.
Multiplying the mole ratios by two to get whole number, the empirical formula
- Find the mass of the empirical unit.
10(12.00) + 7(1.008) + 2(16.00) = 159.06 g/mol
- Figure out how many empirical units are in a molecular unit.
- Write the molecular formula.