When a sample of a compound in the vitamin D family was burned in a combustion analysis, 5.983 mg of the compound gave 18.490 mg of CO2 and 6.232 mg of H2O. This compound was found to have a molecular mass of 399. What is the molecular formula of this compound? Put your answer in form of CxHyOz.

We calculate it as follows:

Moles CO2 = 0.01849 g / 44 = 0.000420

Mass C = 0.000420 x 12 = 0.00504 g

Moles H = 2 x 0.006232 / 18 = 0.000692

Mass H = 0.000692 g

Mass O = 0.005982 - (0.00504 + 0.000692) = 0.00025

Moles O = 0.00025 / 16 = 0.0000156

C 0.000420

H 0.000692

O 0.0000156

divide each by the smallest value, giving you the chemical formula as:

C27H44O