How molarity calculator works
Molarity is the chemist's standard concentration: moles of solute per litre of solution, written M = n / V. The mole counts particles — one mole is 6.02214076 × 10²³ of them — so molarity says how many particles sit in every litre, which is what reaction stoichiometry actually cares about. The calculator takes moles directly, or a mass plus a molar mass and converts through n = m / Mᵣ, then divides by the volume to give molarity in mol/L.
A worked example makes the two-step rhythm visible. Dissolve 58.44 g of sodium chloride — one mole, since NaCl's molar mass is 58.44 g/mol — in water and make the volume up to 1.00 L: the result is exactly 1 M. Use only 5.844 g (0.1 mol) in 500 mL (0.5 L) and the concentration is 0.1 / 0.5 = 0.2 M. The tool handles the millilitres-to-litres division automatically, and reports mM and µM alongside because biologists and pharmacologists work a thousand- and a million-fold below molar: 0.25 M is 250 mM is 250,000 µM.
Scope note: this tool computes the molarity of what you dissolve, not how to get from one concentration to another — dilution follows the separate relation C₁V₁ = C₂V₂ and deserves its own calculator. Also remember that molarity is defined per litre of final solution, not per litre of water added: dissolve the solute first, then bring the volume up to the mark. Molar masses for elements come straight from the periodic table; for compounds, sum the atoms (water: 2 × 1.008 + 16.00 = 18.016 g/mol).