Molarity: Moles per Liter—The Most Common Concentration Unit
Molarity (M) is the number of moles of solute per liter of solution. It is the primary concentration unit in chemistry because it directly relates to the number of molecules, simplifying stoichiometry, reaction calculations, and laboratory work. A 1 M solution contains exactly 1 mole of solute dissolved in 1 liter of solution.
Why This Chemistry Calculation Matters
Why: Molarity is essential for preparing solutions, running reactions, and interpreting analytical results. It enables direct stoichiometric calculations without conversion factors.
How: Molarity is calculated by dividing moles of solute by liters of solution. For mass-based inputs, convert mass to moles using molar mass first. The dilution equation M₁V₁=M₂V₂ preserves moles when diluting.
- ●Physiological saline is 0.154 M NaCl; concentrated HCl is ~12 M.
- ●For dilute aqueous solutions, molarity ≈ molality.
- ●Always use volumetric flasks for accurate solution preparation.
Sample Examples
🧪 Basic Molarity
Calculate molarity from 0.5 mol NaCl in 250 mL solution
⚖️ Mass to Molarity
5.85 g NaCl dissolved in 500 mL - what is the molarity?
💧 Dilution M₁V₁=M₂V₂
Dilute 6M HCl to make 100 mL of 0.1M solution
🏥 0.9% Saline (Medical)
Prepare 1 L of physiological saline (0.154 M NaCl)
🔬 1M Tris Buffer
How much Tris base to make 500 mL of 1M solution?
📊 Serial Dilution
10-fold dilution of 1M stock to make 50 mL
🍬 5% Dextrose (D5W)
Prepare 1L of 5% glucose solution for IV
Enter Values
⚠️For educational and informational purposes only. Verify with a qualified professional.
🔬 Chemistry Facts
Molarity is temperature-dependent because volume changes with temperature.
— IUPAC
The dilution equation M₁V₁=M₂V₂ works because moles of solute remain constant.
— IUPAC
Blood glucose (~5 mM) and seawater NaCl (~0.5 M) span many orders of magnitude.
— NIST
1 M = 1000 mM = 1,000,000 μM; use scientific notation for very dilute solutions.
— IUPAC
What is Molarity?
Molarity (M) is the most common unit of concentration in chemistry, defined as the number of moles of solute per liter of solution. It tells you how many molecules of a substance are dissolved in a given volume, making it essential for stoichiometry, reaction calculations, and laboratory work.
The Definition
Molarity (M) = moles of solute / liters of solution. A 1M solution contains 1 mole of solute per liter.
Why Use Molarity?
Molarity directly relates to the number of molecules, making stoichiometric calculations straightforward.
- Easy reaction stoichiometry
- Standard lab unit
- Simplifies dilutions
Common Examples
From blood glucose (5 mM) to concentrated acids (12-18 M), molarity spans many orders of magnitude.
- Saline: 0.154 M NaCl
- HCl conc.: ~12 M
- Blood glucose: ~5 mM
How to Calculate Molarity
Molarity calculations involve three key relationships: the definition (M = n/V), the molar mass conversion (n = m/MW), and the dilution equation (M₁V₁ = M₂V₂). Master these and you can solve any concentration problem.
📐 Calculation Methods
From Moles and Volume
If you have 0.5 mol in 250 mL (0.25 L):
M = 0.5 / 0.25 = 2.0 M
From Mass and Volume
5.85g NaCl (MW=58.44) in 100 mL:
M = 5.85 / (58.44 × 0.1) = 1.0 M
The Dilution Equation: M₁V₁ = M₂V₂
The dilution equation states that the moles of solute remain constant when you dilute a solution. This simple relationship lets you calculate any dilution in the lab.
M₁
Initial molarity
V₁
Initial volume
M₂
Final molarity
V₂
Final volume
When to Use This Calculator
Lab Preparation
Calculate how much solute to weigh for a specific concentration.
Dilutions
Prepare working solutions from concentrated stocks using M₁V₁=M₂V₂.
Stoichiometry
Calculate reactant amounts for reactions in solution.