An unknown substance has the composition of 74.97% C, 7.4% H and 17.6% O. The compound has a molar mass of 272.3 g/mole. What is the molecular formula?

1 Answer
Apr 23, 2018

We gots...C_17H_20O_3

Explanation:

As with all these problems, we assume a 100*g mass, and interrogate the molar quantities in order to assess the empirical formula:

"Moles of carbon"=(74.97*g)/(12.011*g*mol^-1)=6.24*mol

"Moles of hydrogen"=(7.4*g)/(1.00794*g*mol^-1)=7.34*mol

"Moles of oxygen"=(17.6*g)/(16.00*g*mol^-1)=1.10*mol

Note that normally, you would NOT be given %O...oxygen is a difficult element to analyze... We divide thru each molar quantity by the LOWEST molar quantity...i.e. divide thru by moles of oxygen to get a trial empirical formula of...

C_((6.24*mol)/(1.10*mol))H_((7.34*mol)/(1.10*mol))O_((1.10*mol)/(1.10*mol))-=C_(5.67)H_(6.67)O...but we want integral numbers... and so we multiply by THREE...

3xxC_(5.67)H_(6.67)O-=C_17H_20O_3

Now the "molecular formula" is a "WHOLE NUMBER MULTIPLE" of the "empirical formula"...and clearly the multiple is one...i.e. the molecular mass of C_17H_20O_3-=272.2*g*mol^-1