Question

Vanillin, the primary flavoring component in vanilla, is a molecule that contains the elements carbon, hydrogen, and oxygen. Complete combustion of a 0.420 g sample of vanillin produced 0.972 g carbon dioxide and 0.199 g of water.
Find the empirical formula for vanillin.

Answer 1

Empirical formula is the least coefficient formula, that is

CxHyOz

we must find x,y,z via gravimetry.

Typically we do this relating to moles of C,H,O

mol of CO2 = mass of CO2/MW of CO2 = (0.972)/(44) = 0.0220mol of CO2

1 mol of CO2 = 1 mol of C ---> 0.0220mol of CO2 = 0.0220 mol of C

mol of H2O = mass of H2O/MW of H2O= (0.199 )/(18) = 0.011055 mol of H2O

1 mol of H2O= 2 mol of H ---> 0.011055 mol of H2O= 0.011055 *2 = 0.02211 mol of H

Now... mol of Oxygen:

Mass of O = Mass of sample - Mass of C - Mass of H

Mass of C = mol of C * MW of C = 0.0220 *12 = 0.264g

Mass of H = mol of H * MW of H = 0.02211 *1= 0.02211 g

Mass of O = 0.420 - 0.264 -0.02211 = 0.13389 g of O

mol of O = mass of O / MW of O = (0.13389)/(16) = 0.008368 mol of O

Ratios:

C/O = 0.0220 /0.008368 = 2.67

H/C = 0.0220 /0.02211 = 1

8/3 = 2.67

then

C =8, H = 8, O = 3

C8H8O3