A compound found in crude oil consists of 93.71% C and 6.29% H by mass. The molar mass of the compound is 128 g/mol. What is its molecular formula?
To determine the molecular formula of the compound, we first need to find the empirical formula based on the given percentages of C and H.
1. Calculate the moles of C and H:
Assume we have 100g of the compound.
- 93.71g of the compound is C
- 6.29g of the compound is H
Calculate the moles of C and H:
moles of C = 93.71g / 12.01g/mol = 7.81 mol
moles of H = 6.29g / 1.008g/mol = 6.23 mol
2. Determine the empirical formula:
Divide the moles by the smallest number of moles to get the ratio:
C: 7.81 / 6.23 ≈ 1.25
H: 6.23 / 6.23 = 1
So the empirical formula is CH₂
3. Calculate the molar mass of the empirical formula:
Molar mass of CH₂ = 12.01 (C) + 2(1.008) (H) = 14.026 g/mol
4. Calculate the ratio of the molar mass given to the molar mass of the empirical formula:
Ratio = 128 g/mol / 14.026 g/mol ≈ 9
5. Multiply the subscripts of the empirical formula by the ratio obtained:
Empirical formula × 9 = C₉H₁₈
Therefore, the molecular formula of the compound is C₉H₁₈.