As an intro to moles, we will learn how to solve the 2 simplest types of mole
conversion problems. A mole is 6.02 x 1023 atoms or molecules. This is
Avogadro's number.
The first type of problem we will learn to solve is moles ↔ atoms. When
converting from moles to atoms or atoms to moles you always use
𝟔.𝟎𝟐 𝒙 𝟏𝟎
𝟐𝟑
𝒂𝒕𝒐𝒎𝒔
𝟏 𝑴𝒐𝒍𝒆
as your conversion factor. Remember that you can always flip
your conversion factor if needed.
Sample Problem: You have found 4.64 moles of gold! How many atoms do you
have?
Solution: Always start by writing your known information ( in this case 4.64 moles
of Au). Then choose the correct conversion factor and solve. One nice thing
about mole ↔ atoms problems is that your conversion factor is always the same.
It is always Avogadro's number ( 6.02 x 1023 atoms = 1 mole).
4.64 mol Au x
𝟔.𝟎𝟐 𝒙 𝟏𝟎
𝟐𝟑
𝒂𝒕𝒐𝒎𝒔
𝟏 𝑴𝒐𝒍𝒆
= 2.79 x 1024 atoms Au
We put our moles on the bottom of our conversion factor because we wanted our
answer to be in atoms. So by putting the mole on the bottom that unit cancelled
out with the mol Au that we started with.
What if we know the number of atoms instead of the number of moles to start?
Sample problem: Your chemistry teacher asks you to find out how many moles of
copper are in a penny. Eschewing all scientific methods and reasoning, you
decide to "wing it" and guess that there are 4.32 x 1026 Cu atoms in the penny.
That looks about right anyway. Now that we know how many atoms we have,
how many moles of Cu (copper) are present?
Go through the same steps. Write what we know, use our conversion factor, then
solve.
4.32 x 1026 Cu atoms x
𝟏 𝑴𝒐𝒍𝒆
𝟔.𝟎𝟐 𝒙 𝟏𝟎
𝟐𝟑
𝒂𝒕𝒐𝒎𝒔
= 718 moles Cu
Notice in this problem we had to flip our conversion factor. We started with
atoms, so we needed to put our atoms on the bottom in our conversion factor so
that the atoms would cancel out and our final unit would be moles.
Now let's talk about the second type of problem, moles ↔ mass. For this type of
problem we always use molar mass as our conversion factor. Molar mass can
be found on the periodic table for each element. So each conversion factor will
be different. The molar mass for each element is listed just below the symbol on
the periodic table. For example, the molar mass of gold (Au) is 196.97.
So 1 mole Au = 196.97 grams Au
Sample problem: You have found 4.64 moles of gold! What is the mass of the Au
(gold) in grams?
Write your known info, use a correct conversion factor, then solve.
4.64 mole Au x
𝟏𝟗𝟔.𝟗𝟕𝒈 𝑨𝒖
𝟏 𝒎𝒐𝒍𝒆 𝑨𝒖
= 914g Au
------- remember those sig figs
Since we started with moles, we put our moles on the bottom of our conversion
factor so that the moles would cancel, leaving us with grams.
What if we start with mass instead of moles?
Sample problem: Since you are a responsible recycler, you have collected 692.4g
of aluminum. How many moles of Al (aluminum) do you have?
Remember you always get molar mass from the periodic table.
1 mole Al = 26.98g Al
692.4g Al x
𝟏 𝒎𝒐𝒍𝒆 𝑨𝒍
𝟐𝟔.𝟗𝟖𝒈 𝑨𝒍
= 25.66 mol Al
We flipped our conversion factor from the last problem, since we needed our
grams on bottom to cancel out with the grams we started with.
So that's our first mole problems. Many more to come!!!