🧪Molarity Calculator
Calculate molarity (molar concentration) of a solution, or solve for mass, molecular weight, or volume. Enter any three values to find the fourth.
Prefer to skip the form? Scroll down and Ask AI Instead. Just describe your situation and let AI handle the math for you in seconds.
Molarity (mol/L)
0.855578
✦ Ask AI Instead
Molarity Calculator
Molarity (M) is the most common way to express solution concentration in chemistry: moles of solute per litre of solution. A 1 M NaCl solution contains 58.44 g of NaCl per litre. This calculator solves for any one of the four variables — mass, molecular weight, volume, or concentration — from the other three.
Formula: M = mass ÷ (MW × V) | n = mass ÷ MW | M = n ÷ V
Preparing solutions of known concentration is one of the most common tasks in chemistry laboratories, biology research, pharmaceutical manufacturing, and industrial processes. Molarity is the standard concentration unit because it directly describes the number of reactive molecules per unit volume — which is what matters for chemical reactions, titrations, and enzymatic assays.
The Molarity Formula Explained
The relationship between molarity, mass, molecular weight, and volume follows from the definition of a mole:
- Step 1 — Find moles: n = mass (g) ÷ molecular weight (g/mol)
- Step 2 — Find molarity: M = n ÷ V (L)
- Combined: M = mass ÷ (MW × V)
Rearranging gives you the other three forms: mass = M × MW × V; MW = mass ÷ (M × V); V = mass ÷ (M × MW).
Common Molecular Weights for Reference
| Compound | Formula | MW (g/mol) |
|---|---|---|
| Sodium chloride (table salt) | NaCl | 58.44 |
| Water | H₂O | 18.015 |
| Hydrochloric acid | HCl | 36.46 |
| Sodium hydroxide | NaOH | 40.00 |
| Sulfuric acid | H₂SO₄ | 98.08 |
| Glucose | C₆H₁₂O₆ | 180.16 |
| Ethanol | C₂H₅OH | 46.07 |
Molarity vs Molality vs Normality
Three concentration units are commonly encountered in chemistry:
- Molarity (M, mol/L): Moles of solute per litre of solution. Temperature-dependent because liquid volume changes with temperature. The most widely used unit in laboratory chemistry.
- Molality (m, mol/kg): Moles of solute per kilogram of solvent. Temperature-independent because it is based on mass, not volume. Used in colligative property calculations (boiling point elevation, freezing point depression).
- Normality (N, eq/L): Equivalents of solute per litre of solution. Used in acid-base and redox titrations where one mole of a substance may donate or accept more than one proton or electron. For a diprotic acid like H₂SO₄, a 1 M solution is 2 N.
Dilution Calculations
When you dilute a stock solution, the number of moles of solute stays constant. The dilution formula is: C₁ × V₁ = C₂ × V₂, where C₁ and V₁ are the initial concentration and volume, and C₂ and V₂ are the final concentration and volume after dilution. To make 500 mL of 0.1 M HCl from a 12 M stock: V₁ = (0.1 × 0.5) ÷ 12 = 0.00417 L = 4.17 mL. Measure 4.17 mL of concentrated acid and dilute to 500 mL total volume.
Practical Tips for Solution Preparation
- Always add acid to water — never water to acid — to prevent dangerous exothermic splashing.
- Use a volumetric flask for precise molar solutions — these are calibrated to a specific volume at a specific temperature.
- Dissolve the solute in a smaller volume of solvent first, then transfer to the volumetric flask and dilute to the mark.
- Allow solutions to cool to room temperature before reading the final volume in a volumetric flask, as hot solutions occupy more volume.
Frequently Asked Questions
What is molarity and how is it different from other concentration units?
Molarity (M) is the number of moles of solute dissolved per litre of solution. It is temperature-dependent because liquid volume changes with temperature. Molality (mol/kg) uses mass of solvent instead of volume of solution and is temperature-independent. Normality (eq/L) counts chemical equivalents rather than moles, which matters for polyprotic acids and redox reagents. Molarity is the standard for most general laboratory work.
How do I prepare 500 mL of a 0.5 M NaCl solution?
Calculate the mass of NaCl needed: mass = M × MW × V = 0.5 mol/L × 58.44 g/mol × 0.5 L = 14.61 g. Weigh 14.61 g of NaCl on an analytical balance, dissolve it in approximately 400 mL of distilled water in a 500 mL volumetric flask, then dilute to the 500 mL mark and mix thoroughly.
What is the molecular weight of a substance and where do I find it?
Molecular weight (or molar mass) is the sum of the atomic masses of all atoms in one molecule, expressed in g/mol. You can calculate it from the molecular formula using this website's Molecular Weight Calculator, or look it up in a chemical database such as PubChem, NIST WebBook, or a standard chemistry reference. Many reagent bottles also list the molecular weight on the label.
How do I make a dilution from a concentrated stock solution?
Use the dilution formula: C₁ × V₁ = C₂ × V₂. For example, to make 200 mL of 0.2 M HCl from a 12 M stock: V₁ = (0.2 × 0.2) ÷ 12 = 0.00333 L = 3.33 mL. Measure 3.33 mL of the 12 M stock solution, add it to approximately 150 mL of water in a 200 mL volumetric flask, then dilute to exactly 200 mL. Always add acid to water, not the reverse.
What does a 1 molar (1 M) solution actually look like in the lab?
A 1 M NaCl solution contains 58.44 g of NaCl dissolved in enough water to make exactly 1 litre of solution. A 1 M NaOH solution contains 40.00 g of NaOH per litre. The physical appearance varies — most molar solutions of common salts look like clear water. Concentrated solutions of colored salts or transition metal compounds may be visibly colored. The concentration tells you about the number of molecules, not the visual appearance.
Why does molarity change with temperature?
Molarity is defined in terms of volume of solution, and liquids expand when heated. A solution prepared at 20 °C will have a slightly different molarity if measured at 37 °C because the same mass of solution occupies a larger volume at the higher temperature. This is usually a small effect for aqueous solutions (water expands about 0.2% per degree Celsius near room temperature), but it matters in precise analytical work where solutions should be prepared and used at the same temperature.
What is the difference between molarity and parts per million (ppm)?
Molarity expresses concentration as moles per litre and is independent of molecular weight. Parts per million (ppm) is mass-based: 1 ppm = 1 mg of solute per litre of solution (for dilute aqueous solutions). To convert ppm to molarity: M = ppm ÷ (MW × 1000). For example, 50 ppm NaCl = 50 mg/L ÷ (58.44 g/mol × 1000 mg/g) = 0.000856 M. Ppm is commonly used for trace contaminants; molarity is used when chemical reactivity and stoichiometry matter.