Question

A compound is composed of 85.64% carbon and 14.36% hydrogen. The compound has a formula mass of 42.08 grams. What is the molecular formula?

Answer 1

C₃H₆

Step-by-step explanation:

Answer 2

• The molecular formula is C₃H₆

Step-by-step explanation:

1) Assume 100 g of substace:

• Carbon, C: 85.64% × 100 g = 85.64 g

• Hydrogen, H: 14.36% × 100 g = 14.36 g

2) Convert the masses in grams to number of moles

• moles = mass in grams / atomic mass

• atomic mass of C: 12.011 g/mol

• atomic mass of H: 1.008 g/mol

Calculations:

• C: 85.64 g / 12.011 g/mol = 7.130 mol

• H: 14.36 g / 1.008 g/mol = 14.25 mol

3) State the ratio of moles.

Divide the number of moles of each element by the least number of moles:

• C: 7.130 mol / 7.130 mol = 1.0 mol

• H: 14.25 mol / 7.130 mol = 2.0 mol

4) Set the empirical formula:

• CH₂

5) Calculate the mass of one mol of the empirical formula:

• CH₂: 12.011 g/mol + 2× 1.088 g/mol = 14.027 g/mol

6) Calculate how many times the mass of the empirical formula is contained in the mass of the molecular formula:

• 42.08 g / 14.027 g = 2.9999 = 3

7) Set the molecular formula:

Multiply each subscript of the empirical formula by the previos ratio: 3

• C: 1 × 3 = 3

• H: 2 × 3 = 6

Conclusion: the molecular formula is C₃H₆