# Relationship between molarity and osmolarity of water

### What Affects the Osmolarity of a Solution? | Sciencing

Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, one-to-one relationship between the molarity and the osmolarity of a solution. Penetrating solutes can diffuse through the cell membrane, causing momentary changes in cell volume as the solutes "pull" water molecules with. concentration wants to study the relationship between osmolality and osmolarity. The conversion factor defined as ratio of the solution molarity and its molality was introduced to convert osmolality to could be considered equal to the water. The concentration of a solute in a solution is expressed as its “Molarity”. Osmolarity is a concept that allows you to determine if water will move from one side of.

So that's easy to do. And then you have, let's say, sodium chloride.

## Osmotic concentration

So you have NaCl. And you have two moles of it. We put in two moles of it into one liter. So you say, you have two molarity of sodium chloride. And finally, you have glucose. And you say, well, glucose-- and you're getting the pattern here. Three moles and becomes obviously same volume, and you have three molarity. So that's pretty straightforward, one, two, three. Now, imagine I actually take a little magnifying glass. I'm going to leave that up.

Take a little bit of that water, and let's say, I zoom in on it. This is where things get really interesting. Let's say I zoom in on this a little bit of water right there, just to get a better look at what's going on.

So I zoom in on it, and I get something like this. Let's see if I can draw it out for you. Oh, my circle is not so neat, but you get the idea. So you zoom in on that little circle, and here's what you might see. I'm going to draw the sodium first. So you might get something like this. And let's draw another sodium over here.

And just to label it, so you know what it is. And it's positively charged. And sodium you positively charged, and we have some chlorides. And I'm not drawing them next to each other on purpose, because you'll see what happens. Even though sodium and chloride started out as partners.

They started out next to each other. The moment they hit water an interesting thing happens. So the second they hit water, you've got H2O.

And oxygen is slightly negatively charged. And let's draw oxygen there. And it's attached to two hydrogens, two little hydrogens like that. And this is your slightly negatively charged oxygen and your slightly positively charged hydrogens. And so that negative oxygen and that positive nitrogen attract each other.

## How is osmolarity related to molarity?

So it's going to line up like that. In fact, you might even get another oxygen over here, line up with its two hydrogens and maybe even another one over here. And you see what's happening is that, these oxygens and the hydrogens are lining up, so that the oxygens can be close to the nitrogen, or to the sodium, I said nitrogen by accident, sorry.

And it happens over here too. Oxygen comes in close to the sodium, because it's got that little negatively-charged part to it-- call it a partial dipole-- and a little bit over here too. So some of that negativelyy-charged oxygen is being attracted to the very positive sodium.

And actually, the opposite is happening over here. Here, you have these slightly positively-charged hydrogen, two of them. And those slight positive charges are attracted to the very negative chloride. So you have another one over there. And let's say, you've got some over here. So you get these little water molecules that are lining up next to sodium and chloride and basically getting between them, so they're not next to each other.

So they basically start acting like their own little particles.

Now, here's the key of osmolarity. Think about individual particles that are affecting the movement of water. And so really, sodium and chloride, they're not acting as one anymore.

They're acting as their own individual particles. And you might be thinking, well, whatever happened to that glucose that was in the water. And that's right there. Let's imagine little glucoses. And I'm drawing them very tiny, although we know that the molecule is actually pretty large. And here's our urea. So we haven't lost our urea and glucose. But the key is that, they're lining up. The water is lining up so that it actually blocks out the sodium from the chloride, separating those two ions from one another, so that they behave as individual particles.

So now, if you're looking at individual particles, how many individual different particles are there in this solution of water that's going to affect the movement of water? So we obviously have glucose. And we have urea. And now we have some sodium and four, we have chloride. So I'm really counting sodium and chloride as two separate things now, because they're separated out by the water. So now, if that's the case, let's go back to our question of molarity.

And I'll write up here osmolarity now, osmolarity.

molarity vs osmolarity vs tonicity

And let's see if we can figure out the osmolarity of each of these things. So what is the osmolarity of urea? Well, for urea, we would say, well, there's still just that one mole in one liter. So that's going to be one osm.

### PeteSmif: Osmolarity

And we could say, well, I'm going to jump to glucose now. And sodium chloride, we'll do last. Glucose, we still have the three moles.

And that's still in one liter. So that's three osms. The number of particles, atoms or molecules in a mole of any substance is given by Avagadro's number which is about or 6 x mol-1 Osmolarity Osmolarity is a measure of the osmotic pressure exerted by a solution across a perfect semi-permeable membrane one which allows free passage of water and completely prevents movement of solute compared to pure water.

Osmolarity is dependent on the number of particles in solution but independent of the nature of the particles. It doesn't matter that the solution contains 1 mole of glucose and 1 mole of sucrose.

This is true of all compounds that dissociate in solution. If two solutions contain the same number of particles they may be said to be iso-osmotic isosmotic with respect to each other. If one solution has a greater osmolarity than another solution it is hyperosmotic with respect to the weaker solution. If one solution has a lower osmolarity than another solution then it is hypo-osmotic hyposmotic with respect to the stronger solution.

Iso, hyper and hypo osmolarity should always be stated with respect to another solution. Tonicity Tonicity is nearly the same as osmolarity. For substance that cannot cross cell membranes, tonicity is practically identical to osmolarity. Tonicity is a measure of the osmotic pressure that a substance can exert across a cell membrane, compared to blood plasma.

Plasma has an osmolarity of about 0. If a substance can cross a plasma membrane, then it cannot exert an osmotic pressure across that membrane. The solute will equilibrate across the membrane instead of forcing water to move. Urea behaves like this, so a 0. In The Real World