|I was wondering if you could please show me how
to do problems with mole. I really don't get it
at all. I have a worksheet that asks how to
determine the mass of a certain amount of atoms
and the equivalents and how many atoms are in
certain amounts of elements. I just really don't
get it and I was wondering if you could please
give me a few examples. Thanks!
|Question Date: 2003-02-12|
Dont worry, I also had a lot of problems with
First things first; pull out your Periodic
Table. Find the atomic weight (probably in
the top left corner.) This Atomic weight is
the weight of 1 mole of atoms of that kind.
For instance look at N (Nitrogen), you will see
the atomic mass is 14.00674 grams. So that means
if you had 1 mole of nitrogen atoms, it would
weight, 14.00674 grams.
So if you have a fraction of a mole, say for
instance, just multiply * 14.00674 grams to
obtain the weight, 7.00337 grams. (I used the
Fraction of a mole * Atomic weight = weight
or mass you have
Now we have to get what fraction of a mole you
have. To start, you need know that all a mole
means is you have 6.022 x 1023 atoms of
that type. So if I tell you I have a mole of
nitrogen, then I have 6.022 x1023
nitrogen atoms. That ugly number is called
Avogadro's Constant, NA. Scientist use NA
just so they don't have to keep writing that long
number over and over again.
To get the fraction of a mole you have, take
the number of atoms you have, and divide it by
6.022 x 1023. (Use equation below):
Number of atoms you have / 6.022 x
1023 = Fraction of a mole
After you do that, plug it into the first above
equation and that should give you your answer.
1 mole = 6.03 x 1023 particles.
One mole of any element has a mass in grams
that is equal to its atomic number, and has
exactly 6.02 x 1023 atoms - however
because the atoms of each element have different
sizes and weights, then the volume that each one
occupies is different. Also, some elements are
solids, liquids, or gases at room temperature, so
they will also occupy different volumes.
The trick to working with moles is to
understand that you are dealing with different
units, and you have to translate between them.
We call this "units factoring" because it
involves setting up ratios of the units and
cancelling... For example: money. The smallest
amount of money you can have is a penny. You
can't have half a penny. But you can translate
every amount of money into its equivalent number
of pennies. The standard is that 1 dollar = 100
pennies. So, 10 dollars = 10 dollars x 100
pennies/dollar = 1000 pennies. Similarly, half a
dollar = 0.5 dollar x 100 pennies/dollar = 50
pennies. And, working backwards, if someone
gives you 50 pennies and asks you how many dollars
you have, you can probably do this in your head,
but let's write it out for the sake of
illustrating the point:
50 pennies x 1 dollar/100 pennies = 50/100 =
0.5 dollar. It works just like this for doing
mole fraction problems.
If one mole of carbon, for example, has a mass
of 12 grams, then 12 grams of carbon contain
(12)(6.03 x 1023 ) atoms. How many
atoms are in 3 grams of carbon? Well... (3
grams of carbon) x (1 mole of carbon/12 grams ) =
3/12 = 1/4 of a mole of carbon.
Then... ( 1/4 of a mole) x (6.02 x
1023 atoms/mole) = approximately 1.5 x
1023 atoms. If you have a compound
like H2O, then:
one mole of water contains 6.02 x
1023 MOLECULES of water. But each
molecule of water contains 2 H and 1 O atom = 3
atoms, so there are approximately 1.8 x
1024 atoms in a mole of water.
In having trouble with the notion of what a mole
is and how to use the concept, you are not alone.
Many people have problems understanding what a
mole is. A mole is a collective term. Just as
one says, a bunch of grapes, a pride of lions, or
better still, a dozen cookies, one could say, a
"mole of atoms", or a "mole of molecules". A mole
is a collection of Avogadro number of things. The
things could be atoms or molecules.
also be horses. A mole of horses would be Avogadro
number of horses (a lot actually !). Avogadro
number is 6.023 x 10(23).
A mole of H (hydrogen) atoms
contains 6.023 x 10(23) H atoms. A mole
molecules contains 6.023 x 10(23)
H2 molecules or
2 moles of H atoms (since each molecules has 2
A mole of CaCO3 (calcium carbonate) has
one mole (Avogadro number) of Ca atoms, one mole
of C atoms and 3 moles of O atoms.
So what is
special about the Avogadro number. It is
simple multiplicative constant that relates atomic
weights to the simpler, more familiar measure of
Look up a periodic table of the
elements. Under each element entry, you will
usually find the atomic mass. For example,
Oxygen, you find the atomic weight is 16.00. This
means that 16.00 g of oxygen has Avogadro number
of oxygen atoms. Carbon is 12.01. So a 12.01 g
sample of carbon has Avogadro number of carbon
atoms. If we take the CaCO3 example, the
formula weight is the sum of the atomic masses. We
find that for CaCO3, the formula weight is:
Ca(40.08) + C (12.01) + 3 O (16.00) = 100.1
of CaCO3 is one mole of
CaCO3. We can prepare it
by reaction, one mole of CaO weighing 56.08 g, with
one mole of CO2 weighing 44.01 g. If we
work with ounces, instead of g, and wished to
related the atomic weight to ounces, then the
value that we would chose for the Avogadro number
would be different.
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