Osmolarity - Online Tutor, Practice Problems & Exam Prep

Osmolarity, often confused with molarity, is a measure of the total concentration of solute particles in a solution. It is defined as the number of moles of ions per liter of solution. To calculate osmolarity, one can use direct calculation by dividing the moles of ions by the liters of solution. Alternatively, osmolarity can be determined from molarity by multiplying the number of ions in the compound by its molarity. Understanding osmolarity is crucial in fields like chemistry and biochemistry, as it influences the movement of water between compartments through osmosis, affecting biological and chemical processes.

Osmolarity

Osmolarity, sometimes referred to as ionic molarity, represents the number of moles of ions per liter of solutions. So 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 one, we have direct calculation of osmolarity. In this first method we use the moles of ions and the leaders of solution with its formula to calculate osmolarity. Now here this is just simply osmolarity=moles of ionsliters of solution, Again pretty similar to molarity=moles of soluteliters of solution.

Now that we've gotten the 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 One.

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 MLS of solution. All right, 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 to the -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. So 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 three chlorines. According to the periodic table, the atomic mass of aluminum is 26.98 grams and that of chlorine is 35.45 grams. Multiply these numbers and 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 one mole of aluminum chloride as within the formula, three chlorines. So that would be 3 moles of chloride ions. So when we multiply everything on top, divide by what's on the bottom, we get 1.3073 moles of chloride ion. Take those moles, plug them up here, and then when we divide we get 2.61 molarity and that would be the molarity of our chloride ions.

Now within our choices here, this has three sigfix. Here this has only one sig figs. But here we're just going to say 2.61 molar because just saying 3 molar I feel is too much of A rounding decision. So we're going to 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 × the molarity of the compound overall.

So this is yet another way that we can isolate osmolarity. Now that we've seen this 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 aisle.

Osmolarity Example 2

Now remember method two we can use osmolarity equals 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.

All right, so we need to find the osmolarity of hydroxide ions. So it's 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 three here, meaning that there are three hydroxides within this parentheses. So that would be three times.

Now the molarity of the original compound gallium hydroxide has a compound has a molarity of 0.350 molar. So now just do three 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 osmolarity of anyone 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 conversion factors to isolate and 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 two 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 one 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 here our given amount where we're starting is 120 liters and we're going to have to utilize conversion factors to get to our end amount. Our end amount, what 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 450 molar, which means 450 moles of calcium phosphate per one liter of solution. That's our first conversion factor. Then 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 three there, which means that I have 3 moles of calcium ions. So then if we multiply everything out, that's on top, I'll be left with 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?

Osmolarity is a measure of the total concentration of solute particles in a solution. Specifically, it refers to the number of osmoles of solute per liter of solution (osmol/L). It's a concept often used in chemistry, biology, and medicine to understand the solute concentration in fluids, such as blood or urine.

In a practical sense, osmolarity is important because it affects the movement of water between different compartments of the body through a process known as osmosis. Osmosis is the movement of water across a semipermeable membrane from an area of lower solute concentration (lower osmolarity) to an area of higher solute concentration (higher osmolarity). This is crucial for maintaining cellular function and overall fluid balance within an organism.

For example, if the osmolarity of blood is too high, water will move out of the body's cells into the blood, potentially causing the cells to shrink and malfunction. Conversely, if blood osmolarity is too low, water will move into the cells, which can lead to swelling and damage. Understanding osmolarity helps in managing and treating various medical conditions that are associated with fluid and electrolyte imbalances.

How do you calculate osmolarity?

To calculate osmolarity, you need to consider the molarity of the solutes in a solution and the number of particles into which each solute dissociates in solution. Here's a step-by-step guide:

1. Identify the solutes: Determine which solutes are present in the solution.
2. Determine molarity: Find the molarity (M) of each solute, which is the number of moles of solute per liter of solution.
3. Dissociation factor (i): Determine the dissociation factor (i) for each solute, which is the number of particles the solute dissociates into. For example, NaCl dissociates into two particles (Na⁺ and Cl^-), so i = 2.
4. Calculate osmolarity: Multiply the molarity of each solute by its dissociation factor to find the osmolarity contribution of each solute. Then, sum these values to find the total osmolarity of the solution.

The formula for osmolarity is:

$\mathrm{Osmolarity} (\mathrm{Osm}/L)=\sum \left(\text{Molarity of solute} \times \text{Dissociation factor}\right)$

For example, if you have a 1 M solution of NaCl, the osmolarity would be:

$1 M \times 2=2 \mathrm{Osm}/L$

What does osmolarity measure?

Osmolarity measures the total concentration of solute particles in a solution. It's expressed as osmoles (Osm) of solute per liter (L) of solution, often written as Osm/L. This concept is particularly important in biology and medicine, as it helps to understand how water moves between different fluid compartments in the body through a process called osmosis.

When you have two solutions separated by a semipermeable membrane (like a cell membrane), water will naturally move from the area of lower osmolarity (less solute, more water) to the area of higher osmolarity (more solute, less water). This movement of water can affect cell volume and function, so maintaining the osmolarity of bodily fluids within a narrow range is crucial for health. For example, the osmolarity of human blood is tightly regulated around 285-295 mOsm/L. Deviations from this range can lead to clinical conditions such as dehydration or overhydration.