Molarity
Definition and meaning of Molarity in chemistry.
Molarity is the most common way to measure the concentration of a chemical solution. It tells you the number of moles of a dissolved substance in one liter of total solution. Chemists use it every day to make specific liquid mixtures in the lab.
In more detail
To find molarity, you divide the moles of solute by the liters of solution. The solute is the solid chemical you want to dissolve. The solution is the final mixed liquid, which usually contains a lot of water.
Chemists write molarity using a capital M, which is read out loud as molar. For instance, a bottle labeled 2.0 M acid is twice as strong as a 1.0 M bottle. Molarity is very popular because it is incredibly easy to use in a lab setting.
You can simply weigh your solid chemical and then add liquid until you reach the volume line on a flask. You do not need to weigh the liquid itself. However, molarity has one major hidden flaw regarding temperature.
Liquids take up more space when they get warm. They shrink and take up less space when they get cold. Because molarity depends on the physical volume of the liquid, the concentration actually changes with temperature.
A mixture that is perfectly 1.0 molar at room temperature will become slightly less concentrated on a hot day. The same number of particles is now spread out over a slightly larger volume. For most basic lab work, this tiny change does not matter much. But for highly precise analytical chemistry, scientists must control the room temperature carefully.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Symbol | Capital M or mol/L |
| Formula | Moles of solute divided by liters of solution |
| Key Weakness | Changes when temperature changes |
| Common Solvent | Pure water |
A 0.5 M liquid solution of sodium chloride (NaCl) is very easy to make. You take 0.5 moles of solid salt, which is about 29.2 grams. You place it in a flask and add pure water until the total mixed volume is exactly one liter.
Frequently asked questions
Why does molarity use liters of solution instead of liters of water?
The solid chemical takes up some physical space when it dissolves. If you added a full liter of water, your final volume would be too large and your math would be wrong.
How do you read the capital M on a chemical bottle?
You pronounce the capital M as molar. A bottle marked 3.0 M HCl is read as three molar hydrochloric acid.
Why does a hot room change the molarity of a solution?
Heat gives the liquid molecules more energy, causing them to spread out. This expands the total volume of the solution, which lowers the molarity number.