Molality - Online Tutor, Practice Problems & Exam Prep

Molality is a measure of solution concentration, defined as the number of moles of solute per kilogram of solvent, making it temperature-independent since it does not involve volume. The formula for molality (m) is:

$m=\frac{\mathrm{moles}\mathrm{of}\mathrm{solute}}{\mathrm{kilograms}\mathrm{of}\mathrm{solvent}}$

Osmolality, also known as ionic molality, considers the number of dissolved particles in a solution. For covalent compounds, this value is typically one, as they do not dissociate into ions. However, for ionic compounds, osmolality is calculated by multiplying the number of ions by the molality of the solution. For example, sodium chloride dissociates into two ions, sodium and chloride, resulting in an osmolality that is double the molality of the solution. Understanding these concepts is crucial for accurately determining solution concentrations, particularly in scenarios where temperature changes are a factor.

Molality Concept 1

Now molality, which is symbolized by lowercase M, is another way to express solution concentration and is temperature independent. Now the reason for this is that the units for molality don't involve volume and as a result it's not going to be affected by increases or decreases in temperature.

Now we're going to say that molality represents the number of moles of solute per kilogram of solvent. So here molality lower case M equals moles of solute over kilograms of solvent. So this is the formula you need to memorize when it comes to the idea of molality.

m = moles   of   solute kilograms   of   solvent

Molality Example 1

A solution contains 24.8g of sucrose which has a molecular formula of C₁₂H₂₂O₁₁ and it's dissolved in 550 grams of water. Calculate the molality of the solution. So remember, molality equals moles of solute divided by kilograms of solvent. Our solute is the amount that's smaller, so 24.8g of sucrose would be our solute, and then the larger amount will be our solvent. So water here is a solvent.

So what I'm going to do first is the easier one. Let's just convert grams to kilograms. So we have here 550 grams of water and remember 1K is 10³ so 1 kilogram is 10³ grams, so that's 0.550 kilograms. Next I need to find the moles of sucrose. So we have 248 or 24.8, sorry, 24.8 grams of sucrose. For every one mole of sucrose, how many grams do we have?

So we have 12 carbons, 22 hydrogens, 11 oxygens. Their combined mass comes out to 342.296. So here grams cancel out and we'll have our moles, which comes out to be 0.07245 moles. So I'm going to take those moles and plug them over here. Moles of sucrose. So when I divide my moles by my kilograms, I'll get my molality.

So here 24.8 has three sig figs, 550.0 has four sig figs. Always go with the least number of sig figs. So we're going to have three sig figs at the end. So this comes out to be 0.132 molal. So this would be our final answer.

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

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

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 that they have is just one. Here they don't break up into multiple particles or ions, we just keep them together as one molecule, so for them it equals one.

Now the OSMO Wilding formula is that osmolality equals the number of ions times the molality of our solution and our 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 molality of solution.

So if we take a look here, we have 2.5 molal of sodium chloride. Sodium chloride is an ionic compound that breaks up into two ions, sodium ion and chloride ions. That's a total number of two ions. So we're going to say here that the osmolality equals 2 because there's two ions times the molality of our solution.

So again, what's 2.5 molal of sodium chloride solution? That would mean that our total ionic molality here would be 5 mole. So that's the approach you need to take when it comes to ionic modality, 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 4 nitrate in 750 grams of water? All right, so we're going to say here that our total ionic molality equals 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 five ions. Because it's ionic, it breaks up into one lead 4 ion plus 4 nitrate ions. OK, 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 for nitrate.

So 0.400 moles of lead, 4 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 solution.

So plug that in here, and when we do that, we're going to get our osmolality equal to 2.6665 mol here. This has three sig figs and four sig figs. So we'll want to answer to have three significant figures. So it comes out to 2.67 mole 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

Here’s what students ask on this topic:

What is molality?

Molality is a measure of the concentration of a solute in a solution, defined as the number of moles of solute per kilogram of solvent. It's expressed in units of moles per kilogram (mol/kg). Unlike molarity, which is the number of moles of solute per liter of solution, molality is not affected by changes in temperature or pressure because it's based on the mass of the solvent, not the volume of the solution.

To calculate molality, you use the formula:

$\mathrm{molality}\left(m\right)=\frac{\mathrm{moles}of\mathrm{solute}}{\mathrm{kilograms}of\mathrm{solvent}}$

For example, if you dissolve 2 moles of sodium chloride (NaCl) in 1 kilogram of water, the molality of the resulting solution would be 2 mol/kg. Molality is particularly useful in scenarios involving colligative properties, such as boiling point elevation and freezing point depression, because these properties depend on the number of solute particles in a solvent and not on the volume of the solution.

How do you calculate molality?

Molality is a measure of the concentration of a solute in a solution, defined as the number of moles of solute per kilogram of solvent. To calculate molality, you can follow these steps:

1. Determine the number of moles of the solute. You can do this by dividing the mass of the solute (in grams) by its molar mass (the mass of one mole of the substance, usually given in grams per mole).

$\text{Moles of solute =}\frac{\text{Mass of solute (g)}}{\text{Molar mass of solute (g/mol)}}$

2. Measure the mass of the solvent in kilograms. Make sure you're using the mass of the solvent, not the total mass of the solution.
3. Divide the number of moles of solute by the mass of the solvent in kilograms to get the molality.

$\text{Molality (m) =}\frac{\text{Moles of solute}}{\text{Mass of solvent (kg)}}$

Remember, molality is different from molarity, which is the number of moles of solute per liter of solution. Molality is often used in situations where temperature changes are involved, as it is independent of temperature, unlike molarity.

What is the difference between molarity and molality?

Molarity and molality are both measures of concentration in chemistry, but they are calculated differently and used in various contexts.

Molarity (M) is defined as the number of moles of solute (the substance being dissolved) per liter of solution. It's an expression of concentration that takes into account the volume of the entire solution, which includes both the solute and the solvent. The formula for molarity is:

$M=\frac{\text{moles of solute}}{\text{liters of solution}}$

Molality (m), on the other hand, is the number of moles of solute per kilogram of solvent. Unlike molarity, molality is not affected by changes in temperature or pressure because it's based on the mass of the solvent, not the volume of the solution. The formula for molality is:

$m=\frac{\text{moles of solute}}{\text{kilograms of solvent}}$

In summary, molarity is concentration based on the volume of solution, while molality is based on the mass of the solvent. Molarity is commonly used in reactions that occur in solutions, especially in aqueous solutions, while molality is often used in scenarios where temperature and pressure vary, such as in boiling point elevation and freezing point depression studies.