How do you calculate molarity?

Divide the moles of solute by the volume of solution in litres: M = moles ÷ litres. For 0.5 mol dissolved and made up to 2 L, M = 0.5 ÷ 2 = 0.25 mol/L.

How do you find molarity from grams?

First convert mass to moles by dividing the grams by the molar mass (moles = grams ÷ g/mol), then divide by the volume in litres. For 9 g of NaCl (molar mass 58.44 g/mol) in 1 L: moles = 9 ÷ 58.44 = 0.154 mol, so M = 0.154 ÷ 1 = 0.154 M.

How many grams do I need to make a solution of a given molarity?

Use grams = molarity × litres × molar mass. To make 500 mL of 0.25 M glucose (molar mass 180.16 g/mol): grams = 0.25 × 0.5 × 180.16 = 22.5 g.

How do you find moles from molarity?

Rearrange the formula to moles = molarity × litres. Multiply the molarity in mol/L by the volume in litres. For 0.25 M in 2 L, moles = 0.25 × 2 = 0.5 mol.

What is the dilution formula C1V1 = C2V2?

It states that the moles of solute are conserved when you dilute: the concentration times volume before dilution equals concentration times volume after. To make 100 mL of 0.5 M from a 2 M stock, V1 = C2V2 ÷ C1 = (0.5 × 100) ÷ 2 = 25 mL of stock topped up with 75 mL of solvent.

What is the difference between molarity and molality?

Molarity (M) is moles of solute per litre of solution, so it depends on volume and shifts slightly with temperature. Molality (m) is moles of solute per kilogram of solvent, which is mass-based and therefore temperature-independent.

Is molarity per litre of solution or per litre of solvent?

Per litre of the final solution — the total mixture of solute plus solvent — not per litre of solvent alone. That is why you dissolve the solute first and then top up to the mark, rather than adding it to a litre of water already measured out.

Chemistry

Molarity Calculator

Find the molarity, mass, or volume of a solution — and dilute a stock — from the relationships M = moles ÷ litres and moles = mass ÷ molar mass. Pick a reagent or type a formula and the answer updates as you go, with every step shown.

Solve for

Compound formula

parsed molar mass ≈ 58.44 g/mol

Molar massg/mol

Target molaritymol/L

Solution volume

Mass of solute

26.298g

0.9 M · Moderate

How it works out

1
Volume in litres
500 mL = 0.5 L
2
Moles = M × litres
0.9 × 0.5 = 0.45 mol
3
Mass = moles × molar mass
0.45 × 58.44 = 26.298 g

moles of solute

0.45 mol

molar mass

58.44 g/mol

Common reagents

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M = moles ÷ litres · moles = mass ÷ molar mass

The basics

What is molarity?

Molarity tells you how concentrated a solution is. It is the number of moles of solute dissolved in each litre of the finished solution — the whole mixture, not the solvent alone. Almost every molarity problem is really one of two ideas joined together: counting moles, and spreading them over a volume.

M = moles ÷ litres

Concentration

Molarity, written M, is moles of solute per litre of solution. More solute in the same volume, or the same solute in less volume, both raise the concentration.

0.5 mol ÷ 2 L = 0.25 mol/L

moles = mass ÷ molar mass

Counting moles

You rarely weigh out moles directly — you weigh grams. Divide the mass by the molar mass of the compound to get how many moles that is before you work out the concentration.

9 g ÷ 58.44 g/mol = 0.154 mol

Method

How to calculate molarity by hand

Three steps take you from a mass on a balance to a concentration on a label.

1
Convert mass to moles
If you start from grams, divide by the molar mass of the compound. If you already know the moles, skip this step.
moles = grams ÷ molar mass
2
Put the volume in litres
Molarity is defined per litre, so convert millilitres, cups, or gallons to litres first — 500 mL becomes 0.5 L.
500 mL ÷ 1000 = 0.5 L
3
Divide moles by litres
The moles spread over the volume give the molarity in mol/L.
M = moles ÷ litres

Worked examples

See it in four common cases

The same relationships solve for whichever quantity you are missing — molarity, grams, or the volume to dilute to.

0.5 mol of NaCl in 2 L of solution

Given n = 0.5 mol, V = 2 L

M = n ÷ V

M = 0.5 ÷ 2

M = 0.25 mol/L

In the lab

Weighing out a solution

The most common real task is the reverse of the textbook one: you know the molarity you want and need to know how many grams to put on the balance. Rearranging gives a single formula.

grams = M × litres × molar mass

Grams from molarity

Multiply the target molarity by the volume in litres to get the moles you need, then by the molar mass to turn those moles back into a weighable mass.

0.25 × 0.5 × 180.16 = 22.5 g

dissolve, then top up

Make up to volume

Dissolve the solute in a little solvent first, then add more until the total reaches the target volume. Adding solute to a full measure of water overshoots the volume and throws the concentration off.

add solvent to the 500 mL mark

Where it shows up

Why molarity matters

One number that drives reaction stoichiometry, medicine doses, and everyday recipes alike.

titration

Analytical chemistry

Titrations compare known and unknown molarities to find an exact concentration — the backbone of quality control and acid–base analysis.

0.9% saline

Medicine

IV fluids, buffers, and drug doses are specified by concentration so the same volume always delivers the same amount of active compound.

brine, brewing

Food & home

Pickling brines, electrolyte mixes, and cleaning solutions are all concentration problems — molarity is the precise version of "a teaspoon per cup".

Quick chart

How many grams for common reagents?

The grams of each everyday reagent you need to weigh out, at four standard molarities and your chosen volume — the fast answer when you already know what you are making.

Making

grams of solute to weigh out, by target molarity

Reagent	g/mol	0.1 M	0.25 M	0.5 M	1 M
NaCl Sodium chloride	58.44	5.844 g	14.61 g	29.22 g	58.44 g
C6H12O6 Glucose	180.16	18.016 g	45.04 g	90.08 g	180.16 g
NaOH Sodium hydroxide	40	4 g	10 g	20 g	40 g
HCl Hydrochloric acid	36.46	3.646 g	9.115 g	18.23 g	36.46 g
H2SO4 Sulfuric acid	98.08	9.808 g	24.52 g	49.04 g	98.08 g
KCl Potassium chloride	74.55	7.455 g	18.6375 g	37.275 g	74.55 g
NaHCO3 Sodium bicarbonate	84.01	8.401 g	21.0025 g	42.005 g	84.01 g
CaCO3 Calcium carbonate	100.09	10.009 g	25.0225 g	50.045 g	100.09 g
CuSO4 Copper(II) sulfate	159.61	15.961 g	39.9025 g	79.805 g	159.61 g
C2H6O Ethanol	46.07	4.607 g	11.5175 g	23.035 g	46.07 g
C2H4O2 Acetic acid	60.05	6.005 g	15.0125 g	30.025 g	60.05 g
NH3 Ammonia	17.03	1.703 g	4.2575 g	8.515 g	17.03 g

grams = molarity × 1 L × molar mass. Dissolve the solute, then top up to the final volume.

Worth knowing

Molarity versus its cousins

Concentration can be measured several ways. Knowing which is which prevents the most common unit mix-ups.

m = moles ÷ kg solvent

Molality

Per kilogram of solvent, not per litre of solution. Because it is mass-based it does not drift with temperature, which is why it appears in boiling-point and freezing-point work.

N = M × valence

Normality

Moles of reactive units per litre. For a diprotic acid the normality is twice the molarity, since each mole supplies two reactive protons.

ppm = mg ÷ L

Parts per million

For very dilute solutions, milligrams of solute per litre. Common in water-quality and environmental reporting where molarity would be an awkwardly tiny number.

% w/v = g ÷ 100 mL

Percent concentration

Grams of solute per 100 mL of solution. Normal saline is 0.9% w/v — convert it to molarity by dividing by the molar mass.

Rules

Things to get right

A handful of habits keep your concentrations honest.

litres, not millilitres

Watch the volume unit

Forgetting to convert millilitres to litres is the single most common molarity error, and it is always off by a factor of a thousand.

solution, not solvent

Volume of the solution

The volume in the formula is the final solution volume after the solute has dissolved, not the volume of pure solvent you started with.

C₁V₁ = C₂V₂

Dilution conserves moles

When you dilute, the moles of solute do not change — only the volume grows, so the concentration falls in exact proportion.

check the molar mass

Use the right formula mass

Hydrates carry water of crystallisation. CuSO₄·5H₂O weighs far more per mole than anhydrous CuSO₄, so include the water when you look up the molar mass.

M is temperature-sensitive

Volume changes with heat

Because molarity is volume-based, a solution warmed up is very slightly less concentrated than the same solution cold — usually negligible, but real.

significant figures

Don't over-report

Your answer is only as precise as your least precise measurement; a balance reading to 0.01 g does not justify six decimal places of molarity.

Pitfalls

Common mistakes

The errors that turn a correct method into a wrong number.

9 g ≠ 9 mol

Skipping the molar-mass step

Grams are not moles. Always divide the mass by the molar mass before you treat it as an amount of substance.

add to the mark

Measuring solvent, then adding solute

Pouring solute into a litre of water already measured out gives more than a litre of solution, so the real concentration is lower than intended.

FAQ

Common questions

Molarity is a measure of concentration equal to the number of moles of solute dissolved per litre of solution. Its unit is mol/L, abbreviated M, so 1 M means one mole of solute in every litre of the finished solution.

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