# Relationship between molarity and osmolality test

### How is osmolarity related to molarity? | Socratic Aug 3, Answer: Osmolarity equals molarity times the van't Hoff i factor. The video below explains the difference between molarity and osmolarity. May 28, To find the osmolarity of a % NaCl solution, you first calculate the molarity of the salt solution and then convert the molarity to osmolarity. MOLARITY and OSMOLARITY. The concentration of a solute in a solution is expressed as its “Molarity”. It tells you how much of a solute is present. The units of.

And now let's move on to the anion. And the trick to the anion is just thinking of it as sodium. It's almost the same as sodium, but just the reverse. So we know that it's going to be We're going to use as the number here.

Because our assumption is that sodium is a positive charge and for every one positive charge, you need one negative charge. So we're going to assume that all the negative charges are coming from these anions. And these would be things like we said, things like chloride or bicarb, something like that. So again, we don't actually get these numbers or even need these numbers, we simply take that and we multiply by 2 and assume that the other half is going to be some anion.

Now we actually have to convert units still. We have to get over to milliosmoles per liter. And so we know that the anion is going to be monovalent and that gets us to millimoles. And we use the same logic as above. We just say, OK, well if that was millimoles and it's still one particle, meaning it's not splitting up when it hits water and going in two different directions, in a sense, having twice the effect, we're going to end up with milliosmoles per liter, just as before.

So this is our second part done, right? So two parts are done. We figured out the sodium and we figured out the anion. Now let's go over to glucose. So let's figure out how to get glucose as units from what the lab gives us, which I'll tell you in just a second, into something more usable. So how do we actually get over to something usable? Let me actually, switch over. Make some space on our canvas.

So let's say we have our glucose here. And the lab calls us and says, hey, we just got your lab result, it was 90 milligrams per deciliter.

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It's actually a very, very common lab value or common range for a glucose lab value. One thing we have to do right away is figure out how to get from milligrams to moles. And you know that this is what glucose looks like. This is the formula for it. So to get the overall weight, the atomic weight, you could say, well, let's take 6, because that's how many carbons we have, times the weight of carbon, which is 12, plus 12, because that's what we have here, times the weight of hydrogen, which is 1, plus 6, times the weight of oxygen.

And that's going to equal-- this is 72, this is 12, and this is 96, and add them all up together, and we get-- So we have atomic mass units per glucose molecule. Which means, if you think back, which means that one mole of glucose equals grams. And since these are way, way bigger than, I mean this is grams, and we're talking about milligrams over here, so I'm going to just switch it down by 1, So one millimole of glucose equals milligrams. All I did was divide by 1, So now I can take this unit and actually use our conversions. I could say, well, let's multiply that by and-- let's say, one millimole rather, one millimole per milligrams, that'll cancel the milligrams out.

And I also have to get from deciliters to liters, right? So I've got to go 10 deciliters equals 1 liter. And that'll cancel my deciliters out. So I'm left with-- and this 10 will get rid of that so I'm left with 90 divided by 18, which is 5 millimoles per liter. The main factor determining urine concentration is the amount of water which is resorbed in the distal tubules and collecting ducts in response to ADH.

In a dehydrated patient with normally functioning pituitary and kidneys, a small volume of highly concentrated urine will be produced. In a patient with fluid overload the opposite will be an appropriate response. Note that there is no reference interval "normal range" for urine osmolality as the interpretation depends on the clinical condition of the patient to determine an appropriate response.

The most commonly used instrument in modern laboratories is a freezing point depression osmometer. This instrument measures the change in freezing point that occurs in a solution with increasing osmolality. Osmolality can be measured in samples of serum gold top tube or heparin plasma lime top tube. Plasma osmolality can also be calculated from the measured components. While there are many equations, a simple one is as follows: 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.

Molarity vs. molality - Lab values and concentrations - Health & Medicine - Khan Academy

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. 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.

### Osmotic concentration - Wikipedia

So that's going to be one osm. 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. And let me make a little bit of space here. And we have now sodium. And I'm going to do that as its own thing.

And we have two moles. I should rewrite this. I've been writing moles, and that's not accurate. Now we're talking about osmoles. So I should write one osmole, three osmoles. You can see how similar the two concepts are. I replaced the words by accident. Here we have two osmoles of sodium in one liter.

And that means that it's two osms. And finally, we have chloride. And that is also going to be two osmoles per liter. So really, when we started with sodium chloride and split up, we generate more osmoles, total osmoles.