Molarity= (moles solute)/(liters of solution)
Concentration in % (w/v) = 100 x (mass solute in g) / (volume solution in mL)
Volume = Mass / Concentration
Or, “dumb math problems Dr. Moss says I gotta know how to do for lab….”
I‟m not going to collect these assignments and grade them.
However, there WILL be at least one “math
question” on each exam.
To answer it you‟ll need to master the skills displayed here.
So I do suggest you
practice these problems, and others like them, until you‟re comfortable with them!
Definitions you‟ll need:
SOLUTE: The molecule or molecules in solution that you care about, such as DNA, a protein, or a salt.
SOLVENT: The liquid that the solute is dissolved in.
IMPORTANT NOTE ON UNITS:
Often beginning science students will do calculations writing down only numbers in their notebooks, without
This is a terrible idea, and will cause you to make lots of mistakes!
KEEP YOUR UNITS!
number you write down should have some units written with it!
Imagine you‟re taking a trip, and you ask
the driver how far you are from your destination.
The driver responds, “Six”.
Does that really help? Six
Or six yards?
IMPORTANT NOTE ON CHECKING YOUR ANSWERS:
Whenever you do any kind of quantitative problem, you should calculate the answer, AND then check to
see if it makes sense.
For instance, if you are “diluting” a 1 molar solution by adding water, and yet you
calculate that the final solution is GREATER than 1 molar, you should be able to notice that you must have
made an error.
If you calculate that you are going to make a solution by adding a kilogram of solute to a
milliliter of water, you must have made an error… You should be able to “picture” a kilogram of solute, and a
milliliter of water, and know that you couldn‟t dissolve the former in the latter.
CONCENTRATION OF A SOLUTION:
This is the amount of solute present in a given volume of
The concentration can be expressed in many different ways, such as moles per liter, mass
per liter, or %.
But most commonly solutions are given in molarity (M), or “molar” [moles per liter].
This is the number of moles of solute per liter of solution: