Mr. Guaca Mole Explains...
What is a Mole?
Chemists constantly deal with chemicals in two different
ways... the chemicals we can see and measure in the laboratory
(like 16 g of CH4 or 32 g of O2, etc.) and the molecules we see in
our heads that move and combine.
The mole provides a bridge between these two areas. The unit
mole was introduced into chemistry around 1900 by Ostwald,
and he originally defined this unit in terms of gram.
...the molecular weight of a substance, expressed in grams, shall henceforth be
called mole [. . . das in Grammen augedruckte [. . .] Molekulargewicht eines
Stoffes soll fortan ein Mol heissen]"
Molecular (or Atomic) Masses
CH4
H
H2
O2
16 u
1u
2u
32 u
Through experiments, we can find the relative mass of molecules. We know that
one molecule of CH4 has a mass of 16 units or 16 times heavier than a hydrogen
atom alone. We know that one molecule of oxygen, O2, has a mass of 32 units.
Knowing the mass of just one molecule is not very helpful in the lab, however,
because one molecule is just too small.
GRAM Molecular (or Atomic) Masses
CH4
H
H2
O2
16 g
1g
2g
32 g
We COULD measure out 16 GRAMS of CH4 or 32 GRAMS of O2. These amounts
are called the molecular mass in grams or the gram MOLEcular mass. One gram
MOLEcular mass of CH4 is 16 g. One gram MOLEcular mass of O2 is 32 g.
The useful thing about these numbers is that when you measure out one gram
MOLEcular mass of something (its molecular mass or molar mass measured in
grams) you have measured out the same number of molecules.
Numbers of Particles in Gram MOLEcular Masses
Substance Gram Molecular Mass
Number of Particles
CH4
16 g
6.02 x 1023
O2
32 g
6.02 x 1023
CO2
44 g
6.02 x 1023
H2O
18 g
6.02 x 1023
16 g of CH4 contains the same number of molecules of CH4 as 32 g of O2 contains
molecules of O2. The reason that one gram MOLEcular mass has twice the mass
as the other is that each molecule of O2 has twice the mass of each CH4.
The analogy is the DOZEN. If you found the mass of a dozen pennies, the mass
would be 24 g (if each penny has a mass of 2 g). If nickels have a mass of 4 grams
each, then a dozen nickels would have a mass of 48 g. 24 g of pennies contains
the same number of coins as 48 g of nickels because each nickel is twice as
massive as each penny.
Coin
Masses of Money
Mass of One Coin Dozen Mass Number of Coins
Nickel
4g
48 g
12 nickels
Penny
2g
24 g
12 pennies
Dime
1.5 g
18 g
12 dimes
Quarters
7g
84 g
12 quarters
Back to CH4 and O2. The usefulness of this comes up when we look at
experimental data. Say that we burn 16 g of CH4 in O2. We would find that 64 g of
O2 is used up. 16 g CH4 + 64 g of O2 react and form CO2 and H2O. 16 g CH4 is 1gram MOLEcular mass of CH4. 64 g of O2 is 2-gram MOLEcular masses of O2...
Since each gram MOLEcular mass of a substance contains the same number of
molecules, then we know that one molecule of CH4 reacts with two molecules of
O2. The gram MOLEcular mass is a bridge between the measurements we made
in lab and the way that individual molecules that we cannot see react.
The only problem with gram MOLEcular mass is that the name is TOOOOO
long. We shorten it down to mole. So... one mole of CH4 is 16 g, and 1 mole of O2
is 32 g. A dozen contains 12. Scientists have found that a mole contains 6.02 x
1023 molecules, that's 602 000 000 000 000 000 000 000 molecules!
2
A certain amount of a substance may be thought of in terms of its mass, its
number of molecules, its number of moles, or even its volume where 1 mole of
any gas at STP always occupies 22.4 L. The mole provides conversion factors
between these different ways of looking at a substance.
Avogadro’s Number Analogies
1) Avogadro's Number compared to the Population of the Earth.
We will take the population of the earth to be six billion (6 x 109 people). We
compare to Avogadro's number like this:
6.022 x 1023 divided by 6 x 109 = approx. 1 x 1014
In other words, it would take about 100 trillion Earth populations to sum up to
Avogadro's number.
2) Avogadro's Number as a Balancing Act.
At the very moment of the Big Bang, you began putting H atoms on a balance and
now, 19 billion years later, the balance has reached 1.008 grams. Since you know
this to be Avogadro’s number of atoms, you stop and decide to calculate how
many atoms per second you had to have placed.
1.9 x 1010 yrs x 365.25 days/yr x 24 hrs/day x 3600 sec/hr = 6.0 x 1017 seconds to
reach one mole
6.022 x 1023 atoms/mole divided by 6.0 x 1017 seconds/mole = approx. 1 x 106
atoms/second
So, after placing one million H atoms on a balance every second for 19 billion
years, you get Avogadro’s number of H atoms.
3