Molarity Calculator
PrintMolarity (M), also known as molar concentration, is the standard unit used in chemistry to measure the amount of a dissolved solute within a given volume of solution. Defined as the number of moles of solute per liter of solution (mol/L), molarity is essential for preparing chemical reagents, conducting titrations, formulating pharmaceutical solutions, and analyzing biological samples.
Our free online Molarity Calculator allows you to solve for any unknown variable in laboratory solution preparations:
- Calculate Molarity (M): Find solution concentration (mol/L, mM, μM, nM, or g/L) from solute mass, molecular weight, and total volume.
- Calculate Solute Mass (g): Determine exact grams of chemical compound to weigh out when making a target molar solution.
- Calculate Solution Volume (V): Determine total liquid volume required to achieve a target molar concentration.
- Calculate Molecular Weight (g/mol): Determine compound molar mass from empirical lab measurements.
Molarity Formulas: Moles & Mass Equations
In theoretical chemistry, molarity is calculated directly from moles of solute (n) and solution volume (V in liters):
Molarity (M) = Moles of Solute (n) ÷ Solution Volume (V)
In practical laboratory work, you rarely measure moles directly on a scale. Instead, you weigh solute mass (m in grams) and use the compound’s Molecular Weight (MW in g/mol). Combining these yields the master lab molarity equation:
| Target Variable | Master Lab Formula | Laboratory Application |
|---|---|---|
| Molarity (M) | M = Mass ÷ (MW × Volume) |
Finding final concentration after dissolving a weighed solid in liquid. |
| Solute Mass (m) | Mass = Molarity × MW × Volume |
Calculating exact grams of solid chemical to weigh out on an analytical balance. |
| Solution Volume (V) | Volume = Mass ÷ (Molarity × MW) |
Determining required final volume when dissolving a fixed mass of chemical. |
| Molecular Weight (MW) | MW = Mass ÷ (Molarity × Volume) |
Determining unknown compound molar mass from experimental concentration. |
Step-by-Step Solution Preparation Examples
Example 1: Calculating Molarity from Weighed Mass
Suppose you dissolve 10 grams of Sodium Chloride (NaCl) in water to make a final solution volume of 500 mL (0.5 Liters). The molecular weight of NaCl is 58.44 g/mol:
1. Convert volume to liters: 500 mL = 0.5 L
2. Apply molarity formula: M = 10 g ÷ (58.44 g/mol × 0.5 L)
3. M = 10 ÷ 29.22 = 0.342 M (0.342 mol/L)
Example 2: Calculating Mass to Weigh Out for Reagent Preparation
Suppose you need to prepare 2 Liters of a 1.0 M solution of Potassium Nitrate (KNO3). The molecular weight of KNO3 is 101.1 g/mol:
1. Rearrange formula for mass: Mass = M × MW × V
2. Calculate: Mass = 1.0 M × 101.1 g/mol × 2.0 L = 202.2 grams
3. Lab Procedure: Weigh 202.2 g of KNO3, dissolve in a small volume of distilled water, and dilute to a final total volume of 2.0 Liters in a volumetric flask.
Concentration Units & Conversion Factors
In chemical and biological laboratories, concentrations are expressed across several magnitude scales:
| Concentration Unit | Abbreviation | Definition in Moles / Liter | Typical Lab Application |
|---|---|---|---|
| Molar | M (mol/L) | 1.0 mol/L | Standard stock chemical solutions (acids, bases, salts). |
| Millimolar | mM (mmol/L) | 10-3 mol/L (0.001 M) | Biochemical buffers, enzyme assays, culture media. |
| Micromolar | μM (μmol/L) | 10-6 mol/L (0.000001 M) | Cell signaling assays, drug screening, metabolite tracking. |
| Nanomolar | nM (nmol/L) | 10-9 mol/L | Hormone measurement, receptor binding affinity (Kd). |
| Grams per Liter | g/L | g/L = Molarity × MW |
Mass concentration in food, environmental testing. |
Chemistry Terminology Disambiguation
To avoid common lab errors, distinguish between these core terms:
- Solute: The chemical substance being dissolved (e.g., salt, sugar, acid).
- Solvent: The liquid medium doing the dissolving (e.g., water, ethanol). Water is known as the universal solvent.
- Solution: The combined homogeneous mixture of solute plus solvent. *Critical: Molarity measures moles per liter of final SOLUTION, not solvent volume alone. Always dilute to the final mark after dissolving the solute.
- Molar Mass vs. Molecular Weight: Molar Mass is expressed in g/mol using IUPAC standard atomic weights. Molecular Weight is technically relative molecular mass (unitless ratio to Carbon-12). For practical molarity calculations, both values are numerically identical.
To calculate molecular weights of complex chemical formulas, use our Molecular Weight Calculator. For solution mass or volume conversions, visit our Density Calculator or Conversion Calculator.
Frequently Asked Questions (FAQ)
What is the difference between Molarity (M) and Molality (m)?
Molarity (M) is moles of solute per liter of solution (mol/L). Molality (m) is moles of solute per kilogram of solvent (mol/kg). Molarity changes slightly with temperature because liquid volume expands/contracts with heat, whereas molality remains temperature-independent.
Why must I add solute before filling to the final volume mark?
Dissolving a solid solute increases the total volume of liquid. If you add 1 Liter of water to 200 grams of salt, the final volume will exceed 1 Liter, resulting in an inaccurate, diluted molarity. Always dissolve the solid in a smaller volume of solvent first, then dilute to the exact 1 Liter mark in a volumetric flask.
How do I convert Molarity (M) to Grams per Liter (g/L)?
To convert Molarity (mol/L) to Grams per Liter (g/L), multiply molarity by the compound’s molecular weight (g/mol):
Grams per Liter (g/L) = Molarity (M) × Molecular Weight (g/mol)
What is Avogadro’s number in molarity calculations?
Avogadro’s number (6.02214076 × 1023) defines one mole. A 1.0 M solution contains exactly 6.022 × 1023 molecules (or formula units) of solute dissolved in every liter of solution.