Question

Aspirin often used as an analgesic to relieve minor aches and pains. Elemental analysis of aspirin gave the following mass percent composition: C 60.00%, H 4.48%, O 35.53%. The molar mass of aspirin is 180.16 g/mol. Find the molecular formula of aspirin.

Answer 1

1. Assume 100 g of the compound is present. This changes the percent’s to grams

     C 60.00% = 60 g

     H 4.48% = 4.48 g

     O 35.53% = 35.53 g

2. Convert the masses to moles

       C = 60.00/12 g/mol = 5

       H = 4.48/1 g/mol = 4.48 = 4.5

       O = 35.35/ 16 g/mol = 2.2

3. Divide by the lowest, seeking the smallest whole-number ratio

      C = 5/2.2 = 2.27

      H = 4.48/2.2 = 2

      O = 2.2/2.2 = 1

4. empirical formula = C_2.25H₂O

5. Empirical formula wright = 12 X 2.25 + 2 X 2 + 1 X 16 = 45

6. Divide the molecule weight by the Empirical formula wright ; 180/45 = 4

7. So the molecular fommula is; C_{(2.25 X 4)}H_{(2 X 4)}O_{(1 X 4)} = C₉H₈O₄