Molecular Formula 2

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?

Strategy:

1. 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 becomes:

C10H7O2

2. Find the mass of the empirical unit.

10(12.00) + 7(1.008) + 2(16.00) = 159.06 g/mol

3. Figure out how many empirical units are in a molecular unit.

(318.31 g/mol) / (159.06 g/mol) = 2.001 empirical units per molecular unit

4. Write the molecular formula.

Since there are two empirical units in a molecular unit, the molecular formula is:

C20H14O4