Concentration, Mass & Volume for Solutions
Updated October 2026
Molarity is the most common way chemists express concentration: it's the number of moles of a dissolved substance per liter of solution, written as M or mol/L. A 1 M salt solution contains one mole of salt in every liter. This calculator solves for whichever piece you're missing — the molarity itself, the mass of solute you need to weigh out, or the volume of solution to prepare — from the values you already know.
It's worth being clear that molarity is not the same as molar mass. Molar mass is a fixed property of a compound — the grams in one mole, set by the periodic table. Molarity is a property of a solution you make, and it changes with how much you dissolve and in how much liquid. You need the molar mass as an input to work out molarity, which is why the two often appear together.
Everything here rests on one chain: mass divided by molar mass gives you moles, and moles divided by volume gives you molarity. Run it forward to find concentration, or rearrange it to find the mass to weigh or the volume to dilute to. That's the entire logic, and the calculator just handles the arithmetic and unit bookkeeping so you don't transpose a number.
The most practical use is the reverse calculation: you know the molarity you want and the volume you need, and you want the mass to weigh out. Multiply the desired molarity by the volume to get moles, then multiply by the molar mass to get grams. Dissolve that mass and make it up to the target volume. This calculator's "solve for mass" mode does exactly this — the everyday task at a lab bench.
Once you have a stock solution, you'll often need a weaker one. Diluting doesn't change the number of moles of solute — only the volume — so concentration drops in proportion to how much you dilute. That principle (C₁V₁ = C₂V₂) is its own calculation; the dilution calculator handles it directly. Molarity is the starting point for all of it.
Suppose you need to prepare 250 mL of a 0.5 M glucose solution and want to know how much glucose to weigh out. This is the most common real bench task, and working it by hand shows each step the calculator performs. First, a unit note that trips people up constantly: molarity is defined per liter, so the 250 mL volume must become 0.250 L before any calculation. Skipping this conversion is the single most frequent molarity error.
With consistent units, the logic runs in two steps. Begin with the moles needed: multiply the target concentration by the volume, 0.5 mol/L × 0.250 L = 0.125 mol. That is how much glucose, in moles, the final solution must contain. To convert moles into a weighable mass, multiply by the molar mass of glucose, which is about 180.16 g/mol. So the mass required is 0.125 mol × 180.16 g/mol ≈ 22.52 g. You would weigh out 22.52 grams of glucose, dissolve it in less than 250 mL of water, and then top the solution up to exactly 250 mL.
That final detail matters more than it first appears. You make the solution up to the target volume rather than adding the solute to a full 250 mL of water, because dissolving a substance changes the total volume. If you added 22.52 g of glucose to a beaker already holding 250 mL of water, the result would exceed 250 mL and the concentration would come out slightly below 0.5 M. Dissolving first and then bringing the total to volume is what keeps the molarity exact. Entering "solve for mass" with these values into the calculator returns the same 22.52 g and the 0.125 mol figure, and pairs naturally with the molar mass calculator if you need the molar mass of a different compound first.
Molarity is the chemist's default concentration unit because it ties directly to the mole, and chemistry happens mole-by-mole. Reactions combine in whole-number ratios of moles, not grams, so expressing concentration in moles per liter lets you predict exactly how much of one solution will react with another. This is the entire basis of titration, the lab technique for finding an unknown concentration by reacting it against a known one until the reaction is just complete. Without molarity, that calculation would be far clumsier.
The unit also clarifies a distinction that causes endless confusion: molarity versus molar mass. Molar mass is a fixed, unchanging property of a compound — glucose is always about 180.16 g/mol, set by its formula and the periodic table. Molarity describes a solution you prepared and changes entirely with how much you dissolved and in how much liquid. The same glucose can give a 0.1 M solution or a 2 M solution depending only on the recipe. Molar mass is an input to the molarity calculation; the two are not interchangeable, and keeping them straight prevents a whole category of mistakes.
A practical workflow point ties the bench tasks together. Laboratories rarely prepare every concentration from scratch; instead they keep a concentrated stock solution and dilute it as needed. Because diluting changes only the volume and not the number of moles of solute already present, the concentration drops in direct proportion to the added solvent — the relationship captured by C₁V₁ = C₂V₂. Knowing the molarity of your stock is the starting point for every dilution that follows, which is handled directly by the dilution calculator. One more reminder worth internalizing: always confirm your volume is in liters and your final solution is brought up to volume, and the rest of the arithmetic takes care of itself.
Always convert your volume to liters before calculating. A volume entered in milliliters will throw the molarity off by a factor of 1000.
Molarity uses the final solution volume, not the solvent you start with — dissolve the solute, then top up to the target volume.
To prepare a solution, use the "solve for mass" mode: it tells you exactly how many grams to weigh for your target concentration and volume.
Keep molarity and molar mass straight in your head — one describes the solution you made, the other is a fixed property of the compound.