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# Molarity, molality, osmolarity, osmolality, and tonicity - what's the difference?

Video transcript

Voiceover: I want to make it super clear, what the difference is
between a lot of these words that sound really similar, but their subtly
different from each other. So, the first one is molarity, and we know that means moles
in one liter of solution. Keeping the numerator the same, but tweaking the denominator
you get molality with an L. Now, molality is moles
and here instead of a liter of solution we said it
was one kilogram of solvent. That was the major difference
between the two of them. Right? So, here you can see
the denominator is just slightly different
between these two words. If you now carry this on, let's say we switch over to this side, and we go to osmolarity. Osmolarity. We keep the same denominator. We don't change that,
so, let me write that in. We say one liter of solution. That's still the same. What changes it the numerator
is now osmoles. Osmoles. You remember osmoles talks about not just what is in the solution, but how things can split apart. We talked about salt as a great example of having a different
molarity from osmolarity. Rounding it out, then you
can imagine that there's a fourth word osmolality. Osmolality is going to be the
same numerator as osmolarity, it's going to be osmoles. Except in the denominator, we're going to pick up that one kilogram per of solvent definition. So, that's osmolality. So, then, switching between these two, you can see the major difference here is just the numerator. Right? The numerator is different
between these two. That's four of the terms, and the last term that we picked
up was tonicity. Tonicity. Where does that fall
into the mix? Tonicity. We've even split it up
into hypotonic solutions, isontonic solutions, and
hypertonic solutions. We said that's kind of
how we think of tonicity, one of those three groups usually. Very broadly speaking, these terms, these four terms are
really a way to define or describe one solution. If you have a solution you can
describe it using these terms. These terms are used for two solutions. If you have two solutions
separated by a membrane, as you probably write
that with a membrane, then you can use these
terms to describe how they are relative to one another. That's the key difference
between these terms. The first four are
really for one solution, and the tonicity terms
describe how you might talk about two solutions
separated by membrane, and keep in mind that all of this stuff, all of it, let me just
make a little bit of space, is really trying to
refer to medical terms. We usually use these in terms of medicine, and in medicine you might have cells with a permeable membrane. Actually, let me make it a
little permeable membrane. This cell is usually going to
be setting in some solution, and that solution could be the blood, or it could be the interstitial
fluid, or some solution. When we talk about these four terms, and especially, tinicity,
we're talking about the relationship between this solution, and this solution inside of cells. That's one way to frame
all these different words that we've talked about.