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Molarity Calculator

Calculators · Added

Two equations relate a mass, a volume and a concentration, and which one you rearrange depends on what you are doing: making a solution, checking what you made, or working out how far a stock will go. This does whichever the inputs call for.

What are you working out?

From the bottle, or from the molar mass converter on this site

From the bottle label. 100 for an analytical-grade reagent.

How to use the molarity calculator

  1. 1Choose what you are solving for.
  2. 2Enter the molar mass — the molar mass converter on this site works it out from a formula.
  3. 3Fill in the two quantities you know.
  4. 4Set the purity if the bottle is not analytical grade; the mass to weigh is corrected for it.

Examples

Making a buffer

Input
58.44 g/mol, 100 mM, 500 mL
Result
Weigh 2.922 g — that is sodium chloride at physiological-ish strength

Checking what you made

Input
5 g of glucose in 250 mL
Result
111 mM — glucose is 180.16 g/mol

About the molarity calculator

Where this sits between the other two chemistry tools

The molar mass converter turns a chemical formula into grams per mole, which is the input this needs. This turns that into a mass, a volume and a concentration. The dilution calculator then solves C₁V₁ = C₂V₂, which is what you use once a stock exists.

The three steps are usually done in that order and the middle one is where the reagent bottle actually gets opened, which is why purity and hydration are inputs here rather than assumptions.

When to make a stock instead

Below about a milligram, an ordinary analytical balance stops being trustworthy: the reading is at the edge of its resolution and static, air currents and the weighing vessel all matter more than the sample does. The answer is not a better balance but a different plan — make a concentrated stock at a mass you can weigh accurately, then dilute it.

Diluting introduces its own error, but a volumetric dilution from a well-made stock is far more repeatable than weighing half a milligram, and it has the practical advantage that the stock can be made once and used for weeks.

Frequently asked questions

Do I dissolve the solid in the target volume?
No, and this is the most common way a molar solution comes out wrong. Molarity is per litre of solution, not per litre of solvent, and dissolving a solid increases the volume of the liquid. Dissolve in less than the target, then make up to the mark in a volumetric flask. Adding solid to a litre of water gives you more than a litre and a solution under strength.
Why does purity matter so much?
Because the balance weighs everything in the bottle, not just the compound you want. At 98% purity you need about 2% more mass to deliver the same number of moles. The correction is small and the mistake is systematic, which is worse than a random one: every solution made from that bottle is out by the same amount in the same direction.
What about hydrates?
A hydrate carries water in its formula weight and you must use the hydrated figure. Copper sulfate pentahydrate is 249.68 g/mol against 159.61 for the anhydrous salt — weighing the anhydrous number out of a pentahydrate bottle gives a solution about 36% too dilute. Check which form the bottle says before taking a molar mass from anywhere.
Is molarity the same as molality?
No. Molarity is moles per litre of solution and shifts with temperature, because the solution expands and contracts. Molality is moles per kilogram of solvent and does not, which is why it appears in colligative property work and in anything done over a temperature range. Where a procedure specifies molality, this is the wrong quantity.

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