In any case it is always critical to recognize that unit factoring is nothing else than the
application of the rule of proportionality and that it should never been used without the
understanding of each step.
If not convinced, try to calculate the number of molecules of insulin in 1 ml of a 2.5 ppb
solution. Insulin is a protein consisting of 21 amino acids. Its molar mass is 5508. g/mol.
There are 273 billions molecules in 1 ml of 2.5 ppb insulin.
Rule of three in chemistry
Amongst the many conversion done by chemists is the conversion between moles and
grams. This is a cornerstone of chemistry because the mole links the macroscopic to the
microscopic. Chemistry deals with atoms and molecules which are entities too small to be
directly measured. Therefore an amount of entities, the mole, was defined to represent the
standard amount. A mole contains 6.022… *1023 entities.
As we will see later the mass of the mole depends on the entity. A mole of lead is heavier
than a mole of copper because the atom of lead is heavier than the atom of copper. This is
a bit like a carton of 12 large eggs is heavier than a carton of 12 small eggs.
It is often necessary to figure out how many moles are in a given mass. This is easy as
long as we know the mass of a mole.
To answer such a question we can follow several approaches. The easiest and most
versatile is the mathematical approach. The other approaches as some time used. We will
discuss these approach with a simple example: how many moles of water are in 1.0 L of
pure water. We know that 1.0 L of water has a mass of 1000.0 g because 1.0 L equals
1000.0 ml and the density of pure water is 1.0 g/1 ml. We also know that the molar mass
of water, some time called molecular weight, but actually the average molar mass, is 18.0
g/mol.
The mathematical approach
If we have pure water the ratio mass/volume is the same regardless of the amount. We
can write an equality of two proportions:
18.0 g/1 mol = 1000.0 g/x mol and find the value of x
x mol = (1000.0 g/18.0g) *1 mol

The logic approach
18.0 g are in 1 mole
1.0 g is in 1 mol/18
1000.0 g are in (1 mol/18.0 )1000.0
The technical approach
Going from gram to mole cancelling units
1000.0 g | 1 mol
=
| 18.0g
All three approaches are obviously equivalent and can be used. However if you use the
technical approach you must understand that you are actually writing an equality of
proportion. Such an understanding is indispensable for the solution of real life problems.
We will mostly use the mathematical approach in this course.
Going from gram to mole cancelling units

7- How to handle problems:
A common myth is to believe that Science is about finding (or memorizing) formulas in
which data will be plugged in. Such a one step linear approach is totally inadequate for
real life situations. We will need to adopt a methodology based on analysis and
deduction.
7-a Students are advised to adopt a standard procedure based on four activities:
Sort
Indicate clearly what needs to be found versus what is being given. Make sure that you
have the correct units
Strategize
The most important and most difficult activity. A strategy consists in the series of steps
leading to the solution. Real life problem cannot be answered with a single formula in

#### You've reached the end of your free preview.

Want to read all 13 pages?

- Fall '10
- unknown
- Chemistry, Scientific Notation, Decimal, decimal point