Osmolarity: Study with Video Lessons, Practice Problems & Examples

Osmolarity, or ionic molarity, quantifies the concentration of ions in a solution, calculated as the number of moles of ions per liter of solution. The formula for osmolarity is $O^s=\frac{\mathrm{moles}}{\mathrm{liters}}$. Alternatively, if the molarity of a compound is known, osmolarity can be derived by multiplying the number of ions in the compound by its molarity: $O^s=\mathrm{number}\mathrm{ions}\mathrm{molarity}$. Understanding these calculations is essential for various applications in chemistry.

Osmolarity

Osmolarity, sometimes referred to as ionic molarity, represents the number of moles of ions per liter of solutions. So, it sounds kind of familiar. We know that molarity represents moles of solute per liters of solution. Now we're looking at solutes that have charges because now we're looking at ions. And we're going to say, when it comes to osmolarity, there are a few different methods that we can employ to answer particular questions. Now in method 1, we have direct calculation of osmolarity. In this first method, we use the moles of ions and the liters of solution with its formula to calculate osmolarity. Now here, this is just simply osmolarity equals moles of ions over liters of solution. Again, pretty similar to molarity, which is just moles of solute over liters of solution. Now that we've gotten a basic understanding of what osmolarity represents, let's click on to the next video and start doing some example questions dealing with osmolarity under method 1.

Osmolarity Example 1

In this example question, it says, calculate the molarity of chloride ions when dissolving 58.1 grams of aluminum chloride in enough water to make 500 ml's of solution. Alright. Even though they don't directly say it, they want you to calculate the molarity of ions. So they want you to find ionic molarity or osmolarity. Here molarity will equal the moles of chloride ions divided by liters of solution. We already have the volume of our solution. We just have to change the 500 milliliters into liters. So remember 1 milli is 10-3 liters, so that comes out to 0.500 liters. So now we have 0.500 liters here on the bottom and we have to find moles of chloride ions.

We're going to take the 58.1 grams of aluminum chloride and we're going to convert those grams of aluminum chloride into moles of aluminum chloride. One mole of aluminum chloride, what is the mass of it? Well, we look on the periodic table. It's composed of 1 aluminum and 3 chlorines. According to the periodic table, the atomic mass of aluminum is 26.98 grams and that of chlorine is 35.45 grams. When we add them together, we get 133.33 grams. That represents the mass of aluminum chloride. So grams of aluminum chloride cancel out. Then we're going to convert moles of aluminum chloride, so 1 mole of aluminum chloride, has within the formula 3Cl- chlorines. So that'd be 3 moles of chloride ions. So when we multiply everything on top, divide by what's on the bottom, we get 1.307 mol of chloride ion. Take those moles, plug them up here, and then when we divide we get 2.61 M.

That would be the molarity of our chloride ions. Now, within our choices here, this has 3 significant figures here. This has only 1 significant figure, but here we're just gonna say 2.61 M because just saying 3 M, I feel, is too much of a rounding decision. So we're gonna say 2.61 is the molarity of our chloride.

Osmolarity

The second method involves determining the osmolarity from the molarity. Now we're going to say if the molarity of a compound is known, then the osmolarity for each of its ions can be determined by osmolarity = number of ions that are within that compound ∗ molarity of the compound overall. So this is yet another way that we can isolate osmolarity. Now that we've seen the second method, click on to the next video and let's take a look at a question where we're asked to calculate the osmolarity of a particular ion.

Osmolarity Example 2

Now remember, method 2, we can use osmolarity equals the number of ions times the molarity of the compound. Here in this example, it says, what is the concentration of hydroxide ions in a 0.350 molar solution of Gallium Hydroxide, which has a formula of Ga(OH)₃. Alright. So we need to find the osmolarity of hydroxide ions. It's the molarity of OH^- ions. It equals the number of hydroxide ions. If we take a look here at the formula, we see that there's a 3 here, meaning that there are 3 hydroxides within this parenthesis. So that would be 3 times, now the molarity of the original compound. Gallium hydroxide as a compound has a molarity of 0.350 molar. So now just do 3 times that value. And when we do that we get 1.05 molar as the osmolarity of hydroxide ions. So, it's as simple as that. If you know the molarity of the compound overall, use that to help you find the molarity of any one of its ions.

Osmolarity Example 3

Method 3 involves the number of ions from molarity. Now we're going to say here problems involving number of ions and molarity can use a given amount and converting factors to isolate an end amount. Now for those of you who haven't watched my videos on dimensional analysis and conversion factors, these terms basically form the foundation for a lot of the calculations you'll see in chemistry. Your conversion factor is basically just when you have 2 different units connected together. That is seen in molarity because molarity itself represents moles per liter, 2 units connected together. Our given amount is just a value that's given to us that has only 1 unit, that would be represented by these liters here. And then our end amount is just simply the value or the amount that we're trying to find at the end.

Now if we look at this question, remember, when we have the word "of" in between numbers, it means that we have to multiply them together. So we're going to say our given amount where we're starting is 0.120 liters and we're going to utilize conversion factors to get to our end amount. Our end amount we're looking for is moles of calcium ions. Now we need to cancel out liters here. We're able to do that by utilizing the molarity given to us. The molarity is 0.450 molar , which means 0.450 moles of calcium phosphate per liter of solution. So that's our first conversion factor. Then the liters cancel out. Now we have to just change moles of calcium phosphate into moles of just calcium ion. So here for every one mole of our entire compound, we can see that from the formula there's a little subscript 3 there, which means that there are 3 moles of calcium ions. So then if we multiply everything out that's on top, I'll be left with 0.162 moles of calcium ion as my final answer.

Which of the following solutions will have the highest concentration of bromide ions?

• A. 0.10 M NaBr
• B. 0.10 M CaBr₂
• C. 0.10 M AlBr₃
• D. 0.05 M MnBr₄

How many milligrams of nitride ions are required to prepare 820 mL of 0.330 M Ba₃N₂ solution?

• A. 120 mg
• B. 320 mg
• C. 560 mg
• D. 760 mg
• E. 7600 mg

How many bromide ions are present in 65.5 mL of 0.210 M GaBr₃ solution?

• A. 7.95x10²² ions
• B. 2.48x10²² ions
• C. 7.95x10²³ ions
• D. 6.50x10²³ ions
• E. 3.91x10²³ ions

Here’s what students ask on this topic:

What is osmolarity and how is it calculated?

Osmolarity, also known as ionic molarity, quantifies the concentration of ions in a solution. It is calculated as the number of moles of ions per liter of solution. The formula for osmolarity is:

$Os=\frac{\mathrm{moles}}{\mathrm{liters}}$

Alternatively, if the molarity of a compound is known, osmolarity can be derived by multiplying the number of ions in the compound by its molarity:

$Os=\mathrm{number}\mathrm{ions}\mathrm{molarity}$

How do you calculate osmolarity from molarity?

To calculate osmolarity from molarity, you need to know the number of ions that the compound dissociates into in solution. The formula is:

$Os=\mathrm{number}\mathrm{ions}\mathrm{molarity}$

For example, if you have a 1 M solution of NaCl, which dissociates into 2 ions (Na⁺ and Cl⁻), the osmolarity would be:

$Os=21=2$

What is the difference between molarity and osmolarity?

Molarity measures the concentration of solute in a solution, expressed as moles of solute per liter of solution. The formula is:

$M=\frac{\mathrm{moles}}{\mathrm{liters}}$

Osmolarity, on the other hand, measures the concentration of ions in a solution. It is calculated as the number of moles of ions per liter of solution. The formula is:

$Os=\frac{\mathrm{moles}}{\mathrm{liters}}$

Osmolarity takes into account the dissociation of compounds into ions, while molarity does not.

Why is osmolarity important in chemistry?

Osmolarity is important in chemistry because it helps in understanding the behavior of ions in solutions, which is crucial for various chemical reactions and processes. It is particularly significant in fields like biochemistry, medicine, and environmental science, where the ionic concentration of solutions can affect biological functions, drug efficacy, and water quality. For example, in medical applications, the osmolarity of intravenous fluids must be carefully controlled to match the osmolarity of blood to prevent cell damage.

Can you provide an example of calculating osmolarity using direct calculation?

Sure! Let's calculate the osmolarity of a solution containing 0.5 moles of NaCl in 1 liter of water. NaCl dissociates into Na⁺ and Cl⁻, giving 2 moles of ions per mole of NaCl. Using the formula:

$Os=\frac{\mathrm{moles}}{\mathrm{liters}}$

We have:

$Os=\frac{0.52}{1}=1$

So, the osmolarity of the solution is 1 OsM.