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Molarity Calculator

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How to use the Molarity Formula & Lab Guide

Molarity (M) is the backbone of solution chemistry. Defined as Moles of Solute per Liter of Solution (mol/L), it allows chemists to create reactions with precise stoichiometry. Whether you are titrating an acid or preparing a buffer, accuracy starts with Molarity.

⚗️ Solution Preparation Protocol

Step 1: Weigh solute (g).
Step 2: Dissolve in a small amount of solvent.
Step 3: Dilute to the final volume mark on the volumetric flask.
Never add water to the full volume mark first!

⚠️ Safety First: Diluting Acids

"Do as you oughta, add acid to water." When diluting strong acids (like 18M Sulfuric Acid), always pour the acid slowly into a larger volume of water. Adding water to acid causes an exothermic splash that can result in severe burns.

🌡️ The Temperature Trap

Molarity changes with temperature because liquids expand/contract. A standard 1.0 M solution at 20°C becomes less concentrated (e.g., 0.998 M) at 30°C due to volume expansion.

The Formula

M = n / V

Molarity vs Molality: The Battle of Precision

Feature Molarity (M) Molality (m)
Definition Moles / Liter of Solution Moles / Kg of Solvent
Temperature Dependency Yes (Volume changes) No (Mass is constant)
Primary Use General Lab Work, Titrations Colligative Properties (Boiling Pt)

The Dilution Equation (M₁V₁ = M₂V₂)

Essential for turning a "Stock Solution" into a working solution.

  • M₁: Initial Concentration (Strong)
  • V₁: Volume of Stock needed (Unknown?)
  • M₂: Final Concentration (Weak)
  • V₂: Final Volume desired

Example: Making 100mL of 0.5M HCl from 2M Stock

(2M)(V₁) = (0.5M)(100mL) → 2V₁ = 50 → V₁ = 25mL

Add 25mL of 2M HCl to 75mL water.

Frequently Asked Questions

Frequently Asked Questions

What is the difference between Molarity (M) and Molality (m)?

**Molarity (M)** is moles per Liter of *solution* (volume changes with temperature). **Molality (m)** is moles per Kilogram of *solvent* (mass doesn't change with temperature). Chemists prefer Molality for high-precision work involving boiling/freezing points.

How do I make a 1M solution?

Dissolve 1 mole of your solute (its molar mass in grams) into a beaker, then add water until the **total volume** reaches exactly 1 Liter. Do *not* just add 1 Liter of water to the solute, as the volume will exceed 1L.

Can Molarity change with temperature?

Yes! Since liquids expand when heated, a solution prepared at 20°C will have a slightly larger volume at 30°C. Since M = mol/L, if L increases, the **Concentration (M) decreases**.