Molarity is moles of solute per litre of solution, and this works out whichever of the three you are missing. Choose what you are solving for, fill in the other two boxes, press Calculate, and you get the answer to four decimal places.
It is for the general chemistry lab report, the titration write-up, and the moment before you make up a solution when you need to know whether the number in your head is right.
One honest detail: the answer always prints four decimal places, whether you like them or not. A third of a mole comes out as 0.3333 M, and 0.3333 x 3 is 0.9999 rather than 1, so trim the output to the significant figures your course actually expects before it goes in a table.
Calculate molarity, moles, or volume of a solution. Enter any two values and select what to solve for.
The formula, in three directions
M = n / V, where M is molarity in mol/L, n is moles of solute, and V is the volume of the finished solution in litres. Rearranged twice more, that gives n = M x V and V = n / M, which is exactly what the three options in the Solve for dropdown are doing.
Pick Molarity and it wants moles and volume. Pick Moles of solute and it wants molarity and volume. Pick Volume (L) and it wants moles and molarity. Reset clears all three boxes and the result. A missing or invalid pair comes back as a plain request for valid values rather than a number, and a volume of zero where you are dividing by it is caught the same way.
The three cases, worked through
Solving for molarity. 0.5 mol of sodium chloride dissolved to 2.0 L of solution. 0.5 / 2.0 = 0.25, and the tool prints Molarity = 0.2500 M.
Solving for moles. How much solute is in 0.25 L of a 1.5 M solution. 1.5 x 0.25 = 0.375, printed as Moles of solute = 0.3750 mol. This is the version you want before you weigh anything out, because it tells you what to put on the balance.
Solving for volume. You have 0.025 mol and you need a 0.5 M solution. 0.025 / 0.5 = 0.05, printed as Volume = 0.0500 L, which is 50 mL. The tool stays in litres because the formula is defined in litres, and this is where most first-year results go wrong: 50 mL and 0.05 L are the same volume and 0.05 and 50 are not the same number.
From a bottle on the balance to a solution in a flask
The calculator never sees your balance, so the mass-to-mole step is yours. Take the molar mass from the bottle or the periodic table, divide the mass you weighed by it, and put that number in the moles box. Sodium chloride is 22.99 + 35.45 = 58.44 g/mol, so 5.844 g is 5.844 / 58.44 = 0.1000 mol, and 0.1000 mol made up to 0.500 L is 0.1 / 0.5 = 0.2000 M.
The mistakes that follow from there are predictable. Millilitres instead of litres is off by a factor of 1000, and it is the single most common error in a first semester. Filling to the mark with the solute already in the flask versus adding solvent to a known volume changes the result, because molarity is per litre of solution, not per litre of solvent. Molarity is also temperature dependent, since the volume of a liquid moves with temperature while the number of moles does not, which is why molality exists for work that needs to hold at more than one temperature. And the tool does not do dilutions: for C1V1 = C2V2, work that out separately.
Lastly, this is arithmetic, not a procedure. How you actually make the solution, what you wear doing it, and what goes in the waste bottle are set by your lab’s own procedure and the safety data sheet for the chemical in front of you, not by a number on a screen.
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Frequently asked questions
What is the difference between molarity and molality?
Molarity is moles per litre of solution and changes with temperature, because the volume does. Molality is moles per kilogram of solvent and does not. For most bench work in a warm lab, molarity is what your procedure asks for.
Can I enter the volume in millilitres?
No. The volume box is litres, so 50 mL goes in as 0.05. Divide millilitres by 1000 first, and check the decimal point before you trust the answer, because this is where a result can be out by three orders of magnitude without looking wrong.
How do I get from grams to molarity?
Divide the mass by the molar mass to get moles, then divide by the volume in litres. Weigh 5.844 g of sodium chloride, take it to 0.500 L, and you have 0.2000 M.
Why does my answer have four decimal places?
Because the tool always formats to four. Your lab report should carry the significant figures of the measurements you started with, which for a balance reading and a volumetric flask is usually two or three, so round before you write it down.
Does it handle dilutions?
No. This solves M = n / V in its three directions only. For a dilution, use C1V1 = C2V2 on the concentrations and volumes of the stock and the target solution.