Have you ever followed a recipe and discovered that you used too much sugar because you measured by “volume” instead of by “grams”? When discussing chemistry and solution concentration, Molality (M) and Molarity (m) are two concepts that both experts and students frequently misunderstand. Although both quantify concentration, but they do so in different ways. Accurate calculations in industry, laboratories, and daily chemical processes depend on understanding when and how to utilize each.
This article will help you to understand the differences between molarity vs molality which will help you boost your confidence when dealing with solutions, regardless of whether you’re a chemist, teacher, or student.
What’s Molarity?
Molarity (M) measures the number of moles (a unit of measurement for the amount of substance)
of solute (the material being dissolved) in one liter of solution.
How to do a mathematical calculation for Molarity?
Molarity Formula:
$$ \textbf{Molarity: } \frac{\text{Moles of Solute}}{\text{Volume of solution (in liter)}} $$
Note: The unit of molarity is moles per liter (mol /liter)
Example of Molarity (M):
Suppose you are in a lab setting, where you are dissolving 3 moles of sodium chloride (NaCl),
commonly known as table salt, in adequate distilled water to prepare one liter of solution. The
molarity of this solution will be:
As per equation you have:
Moles of Solute (NaCl) = 3
Volume of solution (Water) = 1 liter
Here, apply molarity formula:
So, the answer is 3 M (molarity).
Factor affecting molarity:
Molarity (M) is reliant on temperature as variations in temperature can affect a solution’s volume
to expand or contract.
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What’s Molality?
Molality measures the number of moles (a unit of measurement for the amount of substance) of
solute (the material being dissolved) in one kilogram of solvent (a liquid in which solute is
distributed equally).
How to do a mathematical calculation for Molality (m)?
Molality Formula:
$$ \text{Molality (m)} = \frac{\text{Moles of solute}}{\text{Mass of solvent in (kg)}} $$
Note: The unit of molality is moles per kilogram (mol /Kg)
Example of Molality (m):
Assume that 4 moles of sugar are dissolved in 0.5 kilogram of water. This sugar solution has the
following molality:
As per equation you have:
Moles of Solute (Sugar) = 4
Kilograms of solvent (Water) = 0.5 kg
Here, apply molality formula:
So, the answer is 8 m (molality).
Factor affecting molality:
Since molality relies on mass, which is unaffected by temperature changes, it is temperature
independent in contrast to molarity.
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The Differences Between Molarity and Molality
| Parameter | Parameter Molarity (M) | Molality (m) |
| Main Definition | Moles of solute per liter of solution | Moles of solute per kilogram of solvent |
| Defining Units | mol/L | mol/kg |
| Volume vs Mass | Based on volume of solution | Based on mass of solvent |
| Temperature Effect | Temperature dependent | Temperature Independent |
| Common Use | lab concentrations (e.g. lab titrations) | Thermodynamic studies (e.g. freezing and boiling points) |
Conclusion
Even though “molarity” and “molality” sound alike, they’re different! Molarity tells you how many moles of a substance are in 1 liter of the whole solution (like salt in saltwater). Molality tells you how many moles are in 1 kilogram of just the solvent (that’s the liquid part, like water in saltwater).
Key point to recall:
For routine lab solutions use, molarity whereas for temperature variations and thermodynamics use, molality
