Understanding Molarity in the Lab

Molarity (M), or molar concentration, is a fundamental concept for preparing solutions, running reactions, and performing dilutions. Accurate molarity ensures reproducibility and correct stoichiometry.

How to Use This Tool

Enter the mass of your solute (solid or liquid).
Select appropriate mass units (g, mg, µg).
Enter the formula weight (molar mass) in g/mol.
Enter the final solution volume and select units (L, mL, µL).
Results update automatically. Use decimal places for reporting precision.

Worked Examples

Scenario	Mass	FW (g/mol)	Volume	Molarity
1 M NaCl solution (1 L)	58.44 g	58.44	1.000 L	1.000 M
10 mM Tris buffer (100 mL)	121.1 mg	121.14	0.1000 L	0.01000 M
µM protein sample (1 mL)	0.025 mg	25000	1.000 mL	1.000 µM

Common Mistakes Checklist

Using mg but entering volume in L without converting.
Confusing formula weight with molecular weight of hydrate (e.g., CuSO₄·5H₂O).
Using final volume of solvent instead of solution (for concentrated acids/salts).
Not accounting for purity/assay percentage of reagents.

Quick Molarity Unit Converter

Convert between molar (M), millimolar (mM), and micromolar (µM).

Enter Value

From Unit

To Unit

Converted Value

1.000 M

Frequently Asked Questions

What are the accuracy limits of this calculator?

The calculator uses double‑precision floating point (about 15 decimal digits). Accuracy depends on your input precision and correct unit selection. It does not account for temperature‑dependent volume changes or activity coefficients.

When does the simple M = n/V formula fail?

For concentrated solutions, non‑ideal mixing, or when using solvents other than water, apparent molar volume may differ. For precise work with ionic strengths >0.1 M, consider activity corrections.

What are common unit pitfalls?

Mixing µL with g, forgetting that molar mass is in g/mol (not kg/mol), and using sample volume instead of final solution volume. Always double‑check units before proceeding.

Any tips for accurate measurement?

Use calibrated volumetric flasks for final volume, weigh solids on an analytical balance, record temperature, and allow solutions to equilibrate before use. For hygroscopic solids, use a dry box.

Formulas & Assumptions

Basic formula: Molarity (M) = (mass / FW) / volume; all units converted to g, mol, L internally.
Unit conversions: 1 mg = 0.001 g; 1 µg = 1e‑6 g; 1 mL = 0.001 L; 1 µL = 1e‑6 L.
Molarity unit conversions: 1 M = 1000 mM = 1e6 µM.
Constants used: None beyond unit definitions.
Rounding: Results rounded to selected decimal places; intermediate values keep full precision.
Assumptions: Ideal solution, additive volumes ignored, solute fully dissolved and stable.

Molarity Calculator

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