ADVANCED HONORS CHEMISTRY - CHAPTER 7
NAME:
CHEMICAL QUANTITIES
DATE:
EMPIRICAL AND MOLECULAR WKST ANSWERS - V2 PAGE:
1. Your instructor takes drugs. Is this a surprise? At his age only chemicals allow him to function, albeit,
feebly! One drug he consumes everyday is caffeine. (Why do you think he drinks diet Pepsi?) Caffeine
contains 49.5 % carbon, 5.15% hydrogen, 28.9% nitrogen, and 16.5% oxygen by mass. Caffeine has a molar
mass
195 g
of
. What is the molecular formula of caffeine?
mol
First, find the empirical formula:
Element
%
Mass (g) *
Moles **
C
49.5
51.5
4.13
H
5.15
5.15
5.10
N
28.9
28.9
2.06
O
16.5
16.5
1.03
Mole Ratio
4.13
= 4.01 ≈ 4
1.03
5.15
=5
1.03
2.06
=2
1.03
1.03
=1
1.03
Subscript
2
7
7
1
*Since empirical formula is an intensive property, you can take whatever size sample your little heart
desires. When you are given the data in this type of problem it is easiest to assume a 100. g sample so that
the % are changed directly to grams.
** See calculations below
⎛ 49.5 g C ⎞ ⎛ 1 mol C ⎞
⎜⎝
⎟⎠ ⎜⎝ 12.0 g C ⎟⎠ = 4.13 mol C
1
⎛ 5.15 g H ⎞ ⎛ 1 mol H ⎞
⎜⎝
⎟⎠ ⎜⎝ 1.01 g H ⎟⎠ = 5.10 mol H
1
⎛ 28.9 g N ⎞ ⎛ 1 mol N ⎞
⎜⎝
⎟⎠ ⎜⎝ 14.0 g N ⎟⎠ = 2.06 mol N
1
⎛ 16.5 g O ⎞ ⎛ 1 mol O ⎞
⎜⎝
⎟⎠ ⎜⎝ 16.0 g O ⎟⎠ = 1.03 mol O
1
∴ The empirical formula is C4H5N2O
1
Caffeine has an empirical formula of C4H5N2O. Its true molar mass is
195 g
. Calculate the molecular
mol
formula.
Since the molecular formula is a multiple of the empirical formula, the next step is to find out what that
multiple is for this compound and then apply that multiple to the subscripts of the empirical formula.
The Molar Mass (Empirical Mass) for C4H5N2O =
97.1 g
mol
195 g
True Molar Mass
= mol = 2.01 ≈ 2
97.1 g
Empirical Mass
mol
∴ The molecular formula is C8H10N4O2
2 - AHC - Chapter 7 - Empirical and Molecular Worksheet - Answers - V2