Counting Atoms & Molecules: I

Chemical Proportions and Percent Composition
 A chemical formula contains the proportions of the atoms in a
substance: CH4 has 1 carbon atom and 4 hydrogen atoms.
 The molar mass of methane is 16.05 g/mol and 12.01 of those grams
come from the carbon. You may have learned the Law of Definite
Proportions (LDP) in Grade 10 – which states that chemical
compounds ALWAYS have the same proportions of elements by
mass. The MASS Percent of each element in the substance can be
calculated. In methane, the mass percent of C is: 74.82%
 Try page 260 # 1, 3, 5 and the Activity 6.1
Percentage Composition
 When you combine the mass percent for all of the elements in a
compound, you have the percentage composition:
We can calculate Percentage Composition from 2 sources:
Mass Data – try page 264 #11
Chemical Formula – try page 266 #24
Eg for CH4: %C = 12.01 g/ 16.05 g x 100% = 74.82%
% H = 4(1.01 g) / 16.05 g
= 25.18%
Empirical and Molecular Formulas
Compounds: a Combustion Analyzer is a tool that burns an unknown
sample and can determine the percentage of the different elements in the
sample. With this device we can get an Empirical Formula (the simplest
ratio of the formula)
A sample has the following percent composition:
73.13% C
7.38% H
19.49% O
What is the Empirical Formula of the substance?:
 This formula is only the simplest ratio – eg. C2H8 and CH4 have the
same percentage composition...
 We need to use
another tool, a Mass
Spectrometer (page 190) to find the molar mass. Once we know
the molar mass we can find the Molecular Formula
eg. The sample from the question above is found to have a Molar
mass of 164.22 g/mol. What is it?
Eg2 Hydrates: Hydrates have water ‘stuck’ to them. For example:
CuSO4.5H2O - this substance has a molar mass of 249.70 g/mol (you have to
add in the hydrate part). If a hydrate is heated, the water can be driven off,
from the change in mass we can determine the formula of the hydrate.
 A sample of an unknown zinc nitrate hydrate
The sample has a mass 74.37 grams. The sample is heated (this drives off
the water) and the final mass is 47.82 grams. Determine the formula of the