In this screencast we are going to cover the basics of calculation of osmolarity. Osmolarity
is commonly used for the driving force for osmotic pressure and osmotic lysis of cells
so this is an important concept to understand in the context of bioseparations. Now in this
example we have been asked to calculate the concentration, or the grams per liter of sodium
chloride that we would add to deionized water to make a solution that has an osmolarity the same as human
serum. And this particular of question is important in a medical context to make sure
that the solutions we ever add to our body are actually balanced in terms of their salt
concentration and also balanced in terms of osmolarity so that we are not putting our
bodies under osmotic stress by adding fluids. Osmolarity is fundamentally a calculation of
the moles of particles per liter of solvent. It is important to distinguish this from
the concept of molarity which is actually moles of a particular species. Here we are
talking about more generally, particles. These particles could be anything, we could be
looking at ions, we could also be looking at polymers. There is also an example used
in a previous screencast where we actually incorporate the determination of osmotic pressure
as a way of calculating how many particles per liter of solution there are and the molecular
weight of those materials. So there is a lot of different things you can do with that but
fundamentally here we are looking at moles of particles per liter of solvent. So in this
example we are actually given the moles of particles per liter of solvent in human serum.
So that is composed of all sorts of ions, different proteins, all sorts of things in
blood but we have boiled that down to the basic concept that the concentration of particles
per liter of solvent is actually this 0.3 osmolar. So what we want to figure out is how much
sodium chloride we would have to add to balance that osmotic pressure, that osmolarity of the system.
So quite simply, in this problem, all we are asked to do is take this 0.3 osmolar
or 0.3 moles of particles per liter of solvent and actually figure out how many moles of particles there
are in sodium chloride and then convert that over to the grams per liter of sodium chloride that we
need to do. So the fundamental principle or the fundamental question we have is, how many
moles of particles are there per gram of sodium chloride? So to answer that question we fundamentally
have to ask ourselves if we have 1 mole of sodium chloride, what does that turn into upon addition
to solution and we know just from basic salt chemistry that we go from 1 mole of sodium chloride would
actually turn into 1 mole of sodium ions plus one mole of chloride ions. So it would fundamentally
give us 1 mole of NaCl turning into 2 moles of particles and that would be the 1 mole
of sodium and the one mole of chloride. Now an important assumption we are making at this point is
that we have complete dissociation of NaCl. Now in the case of sodium chloride that is a pretty safe
assumption that all of the sodium chloride is going to completely dissociate, but there are special
cases throughout chemistry where that is not true. So in other cases you are going to have
to take into account how much of this particular material dissociates and then use that correction
to account for that. In this case sodium chloride completely dissociates in normal conditions so we can
safely assume the 1 mole of sodium chloride turns into 2 moles of particles. So if we use this information,
incorporate that back to the original calculation we have 0.3 moles of particles per liter in blood,
which we want to translate to 0.3 moles of particles of sodium and chloride ions per liter in this solution
that we are making up. So we need to then convert these moles of particles into moles of sodium chloride
so then we can eventually calculate how many grams of sodium chloride that we actually have.
So from here all we need to do is use our conversion factor that we determined down
here and then finally we have moles of sodium chloride as the basic concept of converting that into
grams of of sodium chloride and we have our solution. And that gives us 8.8 g NaCl per liter of the deionized
water. If we want to compare that to something more practical we can actually do a quick
internet search and see that medically, people actually introduce 0.9% NaCl per liter which
is almost perfect in terms of what we have seen here as our solution.