Molarity
What is the Molarity?
Molarity (also called molar concentration) measures the number of moles of solute dissolved per liter of total solution. It's the most common way concentration is expressed in chemistry, since it directly connects to the amounts used in stoichiometric calculations.
A key detail: V is the total volume of the finished solution, not just the volume of solvent (like water) that was added — dissolving a solute typically changes the total volume slightly.
What Each Variable Means
When to Use It
- Expressing how concentrated a solution is
- Preparing a solution of a target concentration from a known amount of solute
- Converting between moles and volume in stoichiometry problems involving solutions
Step-by-Step Example
Problem: Dissolve 2 mol of NaCl in water to make 0.5 L of solution. What is the molarity?
Moles of solute and total solution volume.
n = 2 mol, V = 0.5 LDivide moles by volume.
M = 2 / 0.5Interactive Calculator
Common Mistakes
Mistake: Using the volume of solvent added instead of the total solution volume.
Fix: V is the volume of the entire finished solution — dissolving a solid solute can change the total volume, so measure the final solution, not just the water poured in.
Mistake: Forgetting to convert grams of solute to moles first.
Fix: If given a mass instead of moles, divide by the substance's molar mass to get moles before applying M = n/V.
Practice Questions
Dissolve 0.5 mol of KCl in 250 mL (0.25 L) of solution. Find the molarity.
How many moles of solute are in 2 L of a 3 M solution?
Hint: Rearrange M = n/V to solve for n.
Frequently Asked Questions
Is molarity the same as concentration by mass?
No — molarity measures moles per liter, while mass concentration measures mass per volume (like g/L). They're related through the solute's molar mass but aren't interchangeable directly.
Does temperature affect molarity?
Slightly — since volume can expand or contract with temperature, the molarity of a solution can shift a small amount as temperature changes, even though the moles of solute stay fixed.