Moving Moles
Chemistry in the Biotechnology Lab
Me Marty.
Me love chemistry!!!
Me hate Math!!!
(http://en.wikipedia.org/wiki/Mole_(animal))
So it’s your first day in your new dream job!
The researcher you work for (AKA your Principle Investigator (PI)) turns to
you and says “Hey we are going to do an experiment tomorrow. Please
make a 1.5M sodium chloride solution. We already have a 6M sodium
chloride stock solution” So…What do you do???
Well you move moles!
Basic ChemistryRemember the following ideas from high school chemistry. Atoms are comprised
of electrons, neutrons and protons. The electrons are negatively charged and
orbit around a nucleus of positively charged protons or protons and neutral
charged neutrons depending on the atom. For example oxygen and hydrogen are
atoms. Atoms are used to build molecules like water, glucose, deoxyribonucleic
acid (DNA), etc. So a molecule is what we called at least two atoms bound
together chemically. The chemical formula for a water molecule is H2O because
water has 2 atoms of hydrogen and 1 of oxygen which are bound together
chemically.
A long time ago, chemists realized that they could not directly count the amount of
chemicals made by chemical reactions. The amounts were just too small. So they
decided to define a new unit they called a “mole”.
The classic definition of a mole is the number of particles in 12 g of your average
run of the mill carbon (6.022x1023particles), which is called Avagadro’s number.
“Particles” here are defined as the smallest individual unit (atoms or molecules)
depending on what with which you are working. In the case of carbon, the
particles would be atoms. But if wanted a mole of glucose the particles would be
molecules. This is because the atoms in glucose are chemically bound to each other
to make the particle.
 So in short, no matter what the substance, 1 mole is always comprised of
6.022x1023 particles be it atoms or molecules.
OK now let’s move some moles:
Your instructor will show you how to properly remove the plastic from the
pipette and measure volumes with it.
You have in front of you a 50 ml tube of glitter in water, a 50 ml tube of water,
several beakers, 10 ml pipettes and pipetor.
Assuming that each piece of glitter represents 1 mole (6.022x1023 molecules) how
would you move on average 35 moles into one beaker/ml of water?
Rules you cannot use anything else but what is provided here.
 you cannot suck-up individual or several pieces of glitter repeatedly and put
them in the beaker provided.
 you have 15 mintues to come-up with a solution.
 The first person to come-up with a viable solution will earn 5 pts on the next
exam!
Questions:
Define accuracy and precision?
What would happen if you did not measure the water accurately or precisely?
Back to your original problem:
Now remember you were asked to “Please make a 1.5M sodium chloride solution
in water.” You already have a 6M sodium chloride stock solution.
You can’t see sodium chloride in the water like you can see the glitter you used
earlier, but you are still moving moles… But in this case you are moving moles of
sodium chloride.
So what does 1.5M mean?
1.5M means 1.5 Molar solution = 1.5 moles/liter of liquid (in this case water), and
of course 6M = 6 moles/L.
So the stock is 6 moles/L and you want 1.5 moles/L.
The first thing you should ask is how much more concentrated is the 6mole/L
stock versus the 1.5 mole/L solution you want.
So:
6/1.5=4
This means that the stock is 4 times more concentrated than you need. It means
that you must dilute the stock 4 times to get the concentration you want, so
the“dilution factor” here is 4.
It also means that you need to do a 1:4 dilution, meaning one part of stock and 4
total parts of solution. To get there you need to make a new solution that is 1 part
stock and add 3 equal parts of water. How you do this depends on how much you
need to make. For example, you could make a total of 4 ml by measuring 1 ml of
stock and 3 ml of water. Or you could make 400 ml by measuring 100 ml of stock
and 300 ml of water.
So really you should ask “how much do you need?” right?
So let’s say your PI says:
“Hey we are going to do an experiment tomorrow. Please make a 200ml of a 1.5M
sodium chloride solution. We already have a 6M sodium chloride stock solution”
What would you do?
Solutionvolume needed/dilution factor=
200ml/4 =50 ml
So you would measure out 50 ml of 6M sodium chloride, and add 150ml of water
to make the needed 200 ml of 1.5M sodium chloride.
Explain what would you do if you needed 8ml?
Explain what would you do if you needed 340ml?
Solved Question:
So how many moles did you move to make 4 ml of 1.5M sodium chloride?
Solution:
Walk it back…..
6 moles/L = 6 mmoles/mL (i.e. divide the numerator and denominator by 1000)
Since you are using 1 ml to make the 1.5M sodium chloride solution, you moved
was 6 mmoles = 6x10-3 moles.
Your Question:
How many moles did you move to make 340 ml of 1.5M sodium chloride?
Additional Questions:
1) “Hey we are going to do an experiment tomorrow. Please make 3 ml of 7.5M
sodium chloride. We already have 100 ml of a 20M sodium chloride stock
solution”
a) Can it be done with the stock solution you have on hand?
b) If so explain how would you do it showing all calculations?
2) “Hey we are going to do an experiment tomorrow. Please make 20 ml of
3.4M sodium chloride. We already have a 1 ml of 20M sodium chloride stock
solution”
a) Should you be making more 20M stock solution?
b) If not explain how would you do it showing all calculations?