Molarity vs. Molality: Understanding the Key Differences

In chemistry, measuring and expressing a solution's concentration accurately is essential. Among the many ways to describe concentration, two of the most commonly used and often confused are molarity and molality. Although the difference between molarity and molality may seem subtle, these two terms represent fundamentally different concepts.

In this article, we will detail the difference between molality and molarity, define their formulae, explain their respective units, and highlight the specific conditions under which one is preferable.

What is molarity?

Molarity, also known as the molar concentration of a solute, is defined by the number of moles of solute present in 1 liter of solution (not in 1 liter of solvent!). Molarity is expressed as $\text{mol/L}$, which is abbreviated as $\text{M}$ (pronounced “molar”).

Molar concentration allows us to establish a link between the volume of a solution and the number of moles (or mass) of a solute. It can be calculated using the following equation:

where:

• $\text{M}$ — Molarity or molar concentration (in moles per liter, mol/L);
• $\mathrm{n_{solute}}$ — Amount of solute (in moles);
• $\mathrm{V_{solution}}$ — Volume of solution (in liters);
• $\mathrm{m_{solute}}$ — Mass of solute (in grams); and
• $\mathrm{W_{solute}}$ — Molar mass 🇺🇸 of solute (in g/mol).

This formula shows that molarity depends on both the amount of solute and the total volume of the solution, which explains why it can vary depending on temperature (as the volume can expand or contract). Want to calculate morality in the blink of an eye? Use our molarity calculator 🇺🇸!

What is molality?

Another way to express the concentration of a solution is by its molality. The molality of a solution corresponds to the moles of solute divided by the kilograms of solvent. Molality is expressed as $\text{mol/kg}$, which is abbreviated as either $\text{b}$ or $\text{m}$ (pronounced “molal”).The molality formula is as follows:

where:

• $\text{b}$ — Molality or molal concentration (in moles per kg, mol/kg);
• $\mathrm{n_{solute}}$ — Amount of solute (in moles 🇺🇸);
• $\mathrm{m_{solvent}}$ — Mass of solvent (in kg);
• $\mathrm{m_{solute}}$ — Mass of solute (in grams); and
• $\mathrm{W_{solute}}$ — Molar mass of solute (in g/mol).

As you can see, molality differs from molarity only in the denominator. While molarity is based on liters of solution, molality is based on kilograms of solvent. Learn more about molality with our molality calculator 🇺🇸.

Here's a comparison table to help you better understand the difference between molal vs. molar concentrations at a glance:

Want to know more? Check out our article: How to Convert Molarity to Molality.

Example #1: Calculating molarity

Problem:
A standard 12-ounce can of soda contains 355 mL of liquid and has 0.133 moles of sucrose (table sugar) dissolved in it. What is the molarity of sucrose in the beverage?

Solution:
To calculate molarity (M), use the following formula:

1. You already know the moles of solute: $\mathrm{Moles\ of\ sucrose = 0.133\ mol}$.

2. Next, convert the volume from milliliters to liters: $\mathrm{355 mL = 0.355 L}$.

3. Now, substitute the values into the molarity formula:

Answer:
The molar concentration of sucrose in the soda is approximately $\mathrm{0.375\ M\ (mol/L)}$.

Example #2: Calculating molality

Problem:
You dissolve 0.133 moles of sucrose in 200 grams of water. What is the molality of the solution?

Solution:
To calculate molality (b), use the following formula:

1. You already know the moles of solute: $\mathrm{Moles\ of\ sucrose = 0.133\ mol}$

2. Next, convert the mass of the solvent from grams to kilograms: $\mathrm{200\ g = 0.200\ kg}$.
3. Now, substitute into the formula:

Answer:
The molality of the solution is $\mathrm{0.665 b\ (mol/kg)}$.

In summary, molarity and molality both measure concentration, but they do so in different ways: one by volume, the other by mass. After reading this article, you now understand that knowing the difference between molal vs. molar concentrations is essential for accurate laboratory work, especially when temperature changes occur.