Molality: Videos & Practice Problems

Ionic molality, or osmolality, measures the concentration of dissolved ions in a solution. For instance, a 0.30 molal sodium chloride solution dissociates into two ions: sodium and chloride. The osmolality is calculated as the product of the number of ions and the molality, resulting in an osmolality of 0.60. Understanding osmolality is crucial for applications in chemistry and biology, particularly in processes like osmosis and cellular transport.

Molality Concept 1

Molality is depicted as moles of solute per kilograms of solvent. Molality, which is lowercase m, here we have moles of our solute, which is a smaller part of our solution, divided by kilograms of the solvent, the larger portion of our solution. Now, this is similar to molarity, which has capital M. It too has moles of solute as the numerator, but the denominator is actually liters of solution. Because of their similarities, there are questions which at times will ask you to interchange between the two. Now, in the same way we can expand molarity, the same approach can be applied to molality.

So let's say that we have 0.30 moles of sodium chloride. What does this translate into? Well, this just means that we have 0.30 moles of the solute, which is sodium chloride, divided by 1 kg of our solvent, which is usually water. Let's say for example that we had 0.25 M of glucose in an aqueous solution. Now, again, 0.25 M. The number itself means that's how many moles we have of the solute. So, it'd be 0.25 moles of glucose. Here we say aqueous solution. Aqueous means that our solvent is water so that would be 1 kg of water.

So the takeaway from this is, remember, if they give you molality of a compound or solution, it just means that number in terms of moles divided by 1 kg of our solvent. This will be important to remember if we're trying to convert molality to let's say molarity or mole fraction or even mass percent. Now that we've learned the basics of molality, click on to the next video and take a look at "Molality".

Molality Example 1

Alright, so ionic molality or osmolality represents the molality of dissolved ions within a solution. Now, for example, if we're given 0.30 molar of sodium chloride, sodium chloride dissociates into sodium ion and chloride ion. That's a total of 2 ions involved. If we want the osmolality, that's equal to the number of ions, which we said was 2, times the molality of the compound, which is 0.30 molal. This will give us an ionic molality or osmolality of 0.60. So remember, when we're talking about osmolality or ionic molality, we have to take into account the number of ions that dissociate within our given solution. Now that you've seen this, move on to the example left on the bottom of the page and see if your answer matches mine.

A solution is prepared by dissolving 43.0 g potassium chlorate, KClO₃, in enough water to make 100.0 mL of solution. If the density of the solution is 1.760 g/mL, what is the molality of KClO₃ in the solution?

• A. 1.99 m
• B. 3.51 m
• C. 2.64 m
• D. 4.70 m

Answer: (C)

The density of a 15.7 M methanol (CH₃OH) solution is 0.858 g/mL. If H₂O is the solvent, what is the molality of the solution?

• A. 44.2 m
• B. 18.3 m
• C. 31.2 m
• D. 23.7 m

Answer: (A)

Molality Concept 2

Osmolality, also called ionic molality, represents the number of dissolved particles in a solution. And we're going to say that when it comes to covalent compounds, the number of ions they have is just 1. Here, they don't break up into multiple particles or ions. We just keep them together as one molecule, so for them, it equals 1. Now the osmolality formula is that osmolality equals the number of ions times the molality of our solution or compound.

The way we approach this is we break up ionic compounds into ions, and we count the total number of ions and multiply by the molality of the solution. So if we take a look here, we have 2.5 mol of sodium chloride. Sodium chloride is an ionic compound that breaks up into 2 ions, sodium ion and chloride ions. That's a total number of 2 ions, so we're going to say here that the osmolality equals 2, because there are 2 ions, times the molality of our solution. So again, it's 2.5 mol of sodium chloride solution.

That would mean that our total ionic molality here would be 5 moles. So that's the approach you need to take when it comes to ionic molality, otherwise known as osmolality.

Molality Example 2

What is the osmolality of total ions in an aqueous solution prepared by dissolving 0.400 moles of lead(IV) nitrate in 750 grams of water? Alright. So, we're going to say here that our total ionic molality equals the number of ions times the molality of the solution. So, we're going to say here that our total number of ions, this breaks up into 5 ions because it's ionic. It breaks up into 1 lead(IV) ion plus 4 nitrate ions.

Okay. So, 5 total ions. So, it's going to be 5 times. Now we need the molality of the solution. Remember, molality equals moles of solute, which in this case would be the lead(IV) nitrate.

So, 0.400 moles of lead(IV) nitrate divided by kilograms of solvent. Our solvent here is water, and we're going to say 750 grams is 0.750 kilograms. So, here, we plug that in, that gives me 0.5333 as my molality for the solution. So, plug that in here, and when we do that we're going to get our osmolality equal to 2.6665 molal. Here, this has 3 significant figures and 4 significant figures, so we'll want our answer to have 3 significant figures.

So, it comes out to 2.67 molal as our osmolality of the total number of ions.

What is the ionic molality of sodium ions in a solution of 25.7 g NaNO₃ dissolved in enough water to make a 150.0 mL of solution? Density of the solution is 1.02 g/mL.

• A. 1.98 m
• B. 2.57 m
• C. 4.74 m
• D. 2.37 m

Answer: (D)

Here’s what students ask on this topic:

What is the difference between molality and osmolality?

Molality (m) is a measure of the concentration of a solute in a solution, defined as the number of moles of solute per kilogram of solvent. Osmolality, on the other hand, measures the concentration of all dissolved particles (ions, molecules) in a solution. It is calculated by multiplying the molality of the compound by the number of particles into which the compound dissociates. For example, a 0.30 molal sodium chloride (NaCl) solution dissociates into two ions (Na⁺ and Cl^-), resulting in an osmolality of 0.60 osmol/kg.

How do you calculate the osmolality of a solution?

To calculate the osmolality of a solution, you need to know the molality of the solute and the number of particles it dissociates into. The formula is:

$\mathrm{Osmolality}=n\times m$

where $n$ is the number of particles the solute dissociates into, and $m$ is the molality of the solution. For example, for a 0.30 molal NaCl solution, which dissociates into 2 ions, the osmolality is:

$2\times 0.30=0.60$ osmol/kg.

Why is osmolality important in biological systems?

Osmolality is crucial in biological systems because it affects the movement of water across cell membranes through osmosis. Cells maintain a specific osmolality to ensure proper hydration, nutrient uptake, and waste removal. An imbalance in osmolality can lead to cell shrinkage or swelling, disrupting cellular function and potentially causing damage. For example, in medical settings, intravenous fluids are often adjusted to match the osmolality of blood to prevent adverse effects on cells.

What is the osmolality of a 0.50 molal solution of potassium sulfate (K₂SO₄)?

Potassium sulfate (K₂SO₄) dissociates into three ions: 2 potassium ions (K⁺) and 1 sulfate ion (SO₄^2-). To calculate the osmolality, use the formula:

$\mathrm{Osmolality}=3\times 0.50=1.50$ osmol/kg.

So, the osmolality of a 0.50 molal K₂SO₄ solution is 1.50 osmol/kg.

How does temperature affect molality and osmolality?

Molality is independent of temperature because it is based on the mass of the solvent, which does not change with temperature. Osmolality, similarly, is not directly affected by temperature since it depends on the number of dissolved particles per kilogram of solvent. However, temperature can influence the dissociation of solutes and the solubility of compounds, indirectly affecting the osmolality if the number of dissociated particles changes.