CHEM 121
Spring 2012
Chap. 4 p.148 #4.86
Answer to Paper HW#2
due 2/21/12
Vanillin, a compound used as flavoring agent in food products, has the following percentage
composition: 64.2%C, 5.26%H and 31.6%O. What is the empirical formula of vanillin?
mass ratio =
64.2 gC
molar ratio =
64.2 gC
=
1 molC
1 mol H
: 5.26 gH
12.01gC
1.008 g H
5.345 mol C
5.345 mol C
1.975
2.71 mol C
8.13 mol C
reduced ratio =
=
=
x3
: 5.26 gH
: 5.218 mol H
5.218 mol H
:
1.975
: 2.64 mol H
: 7.92 mol H
: 31.6 gO
: 31.6 gO
1 mol O
16.00 gO
: 1.975 mol O
1.975 mol O
:
1.975
: 1.00 mol O
: 3.00 mol O
Ans. C8H8O3 Note: Normally, 8.13 mole is still too far from 8 to be rounded to 8.
The textbook made a mistake in the %C. It should have been 63.1%C instead, in which case the
solution to the problem would look like this.
mass ratio =
63.1 gC
molar ratio =
63.1 gC
=
1 molC
1 mol H
: 5.26 gH
12.01gC
1.008 g H
5.253 mol C
5.253 mol C
1.975
2.66 mol C
7.98 mol C
reduced ratio =
=
=
x3
: 5.26 gH
: 5.218 mol H
5.218 mol H
:
1.975
: 2.64 mol H
: 7.92 mol H
: 31.6 gO
: 31.6 gO
1 mol O
16.00 gO
: 1.975 mol O
1.975 mol O
:
1.975
: 1.00 mol O
: 3.00 mol O
This is much more reasonable to be rounded to C8H8O3.
Vanillin has the structure shown below:
O
H
H
C
C
C
C
C
H
H
C
C
OH
O CH3
Does it have the formula C8H8O3?