# What is the apparent relationship between molarity and osmolarity

### Plasma osmolality - Wikipedia The pressure difference produced between the two compart- molarity, mol/L). In practice, the sample (see below) is an apparent osmolarity obtained by. Describe the difference between osmolarity and molarity. This distinction is due to the fact that some solutes dissociate when they dissolve. Let's say you have 4 moles of NaCl in one liter of water: a 4M solution of NaCl. What's the osmolarity, or sum of the molarities of the dissolved.

So in this case, let's say, we have a few boxes. Let's say, we have one box here.

### Molarity vs. osmolarity (video) | Khan Academy

And this box, I'm going to pack it full of this little green particle. And this is called urea. And if you're not sure what urea is, no worries, I'm going to draw it for you.

And it's a molecule that our body makes to get rid of nitrogen. So you have little nitrogens here, two of them. And in between, you have a carbon and an oxygen, so sort of a small molecule. But it's a very useful molecule for helping us package up the nitrogen, so we can urinated it out or get rid of it some other way. Now, I'm going to draw two more boxes. And these boxes, I'm actually going to put something that you may be more familiar with, and that is salt.

So I'm going to draw little sodiums, and next to them, little chlorides.

## Plasma osmolality

So this is sodium chloride. And again, I'm just drawing a few of them. But just remember, because it's in a box I've got an entire mole of each of these things.

So I've got here sodium chloride. And I'll try to keep the color code consistent.

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

And I have two moles of it, so I've got an equal amount in either box. And now I've got three boxes of glucose. I'm going to draw glucose on this side.

So you can see, I'm going from one box of urea, two boxes of sodium, to now three boxes of glucose. I'm going to just draw glucose as little red balls here.

So each little red ball represents a glucose. And just to remind you what glucose looks like, we're going to draw it out as well. So glucose is a little molecule like this with an oxygen. And off of it, you get these little OH groups, so a little OH there option, oxygen and hyrdogen there.

This one is like that. This one goes down. And you have another carbon coming off of it, with an OH as well. So that's your little glucose, and each little red dot represents one of those molecules. So we've got six moles of stuff here. And I'm going to make a little bit of space on this canvas. And we're going to say now, we're going to take our stuff and put it into a liter. So imagine I take a bucket or something here, and this is full of water, one liter of water exactly. So this is my little one liter.

And you're going to take all this stuff, and let's say, dump it in here. So all six moles of stuff go in there. And now, I ask you, tell me the molarity of this stuff. So we have three things. So let's start with urea. What is the molarity of urea? Well, you'd say, well, I have one mole of it. And I have one liter, so one mole per liter equals one molarity. And a big M represents molarity. So that's easy to do. And then you have, let's say, sodium chloride. 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. A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma. Increased osmolarity frequently occurs following illness due to chronic neurotoxic diseases such as Lyme disease. To calculate plasma osmolality use the following equation typical in the US: If the patient has ingested ethanol, the ethanol level should be included in the calculated osmolality: Osmolar gap OG [ edit ] Main article: Osmol gap The osmolar gap is the difference between the measured osmolality and the calculated osmolarity.

The difference in units is attributed to the difference in the way that blood solutes are measured in the laboratory versus the way they are calculated. The laboratory value measures the freezing point depression, properly called osmolality while the calculated value is given in units of osmolarity.

Even though these values are presented in different units, when there is a small amount of solute compared to total volume of solution, the absolute values of osmolality vs. Often, this results in confusion as to which units are meant. For practical purposes, the units are considered interchangeable.