Cortisone consists of molecules, each of which contains 21 atoms of carbon (plus other atoms). the mass percentage of carbon in cortisone is 69.98%. what is the molar mass of cortisone?

Now to solve this problem, we are given the number of atoms therefore we must first convert this into number of moles. We can do this by using the Avogadro’s number. Calculating for number of moles of Carbon:

number of moles Carbon = 21 atoms of carbon (1 mole / 6.022 x 10^23 atoms C)

number of moles Carbon = 3.49 x 10^-23 mol C

Converting this to mass by multiplying the molar mass of C:

mass Carbon = 3.49 x 10^-23 mol C (12 g / mol)

mass Carbon = 4.18 x 10^-22 g

Therefore the mass of one molecule of Cortisone is:

mass of 1 molecule Cortisone = 4.18 x 10^-22 g / 0.6998

mass of 1 molecule Cortisone = 5.98 x 10^-22 g per molecule

Converting this to mass per 1 mol of Cortisone, by using again the Avogadro’s number:

mass of 1 mol Cortisone = (5.98 x 10^-22 g / molecule) (6.022 x 10^23  molecules / mol)

mass of 1 mol Cortisone = 360.10 g / mol = molar mass of cortisone

360.10 g / mol