How do I calculate molarity?

Divide the number of moles of solute by the volume of solution in litres. 58.44 g of sodium chloride made up to 1 litre gives a 1 M solution.

How do I convert grams to moles?

Divide the mass in grams by the molar mass in grams per mole, which you get by summing the atomic masses in the formula.

What is the difference between molarity and molality?

Molarity is per litre of solution and varies with temperature. Molality is per kilogram of solvent and does not, which matters for freezing and boiling point work.

Should I add solvent to the mark or measure it first?

Dissolve in a smaller volume then make up to the mark. Adding solute to a full litre gives more than a litre of solution and a concentration below target.

Molarity Calculator - Moles, Mass and Concentration

Molarity Calculator

Molarity expresses concentration as moles of solute per litre of solution. It is the standard working unit in chemistry, and this calculator converts between mass, moles and concentration.

Formula

M = moles of solute ÷ litres of solution

moles = mass in grams ÷ molar mass
Litres of solution, not of solvent

0.5 M in 250 mL: 0.5 × 0.25 = 0.125 mol, or 7.31 g of NaCl

The Formula

Molarity (M) = Moles of solute ÷ Litres of solution

And to get moles from a weighed mass:

Moles = Mass (g) ÷ Molar mass (g/mol)

Sodium chloride has a molar mass of 58.44 g/mol, so 58.44 g dissolved to make 1 litre gives a 1 M solution.

Solution Volume, Not Solvent Volume

A subtle but important distinction: molarity uses the volume of the finished solution, not the volume of solvent you started with.

Dissolving solid in 1 litre of water gives slightly more than 1 litre of solution, so the concentration comes out below target. The correct method is to dissolve in a smaller volume, then make up to the mark in a volumetric flask.

Molarity, Molality and Temperature

Molarity is moles per litre of solution and changes with temperature, because liquids expand.

Molality is moles per kilogram of solvent and does not, since mass is temperature-independent. That is why molality appears in freezing point and boiling point calculations while molarity dominates everyday bench work.

Quick Reference

Grams needed for one litre at each concentration.
Compound	Molar mass	0.1 M	0.5 M	1 M
NaCl	58.44 g/mol	5.84 g	29.22 g	58.44 g
Glucose	180.16 g/mol	18.02 g	90.08 g	180.16 g
NaOH	40.00 g/mol	4.00 g	20.00 g	40.00 g
H₂SO₄	98.08 g/mol	9.81 g	49.04 g	98.08 g

Frequently Asked Questions

How do I calculate molarity?: Divide the number of moles of solute by the volume of solution in litres. 58.44 g of sodium chloride made up to 1 litre gives a 1 M solution.
How do I convert grams to moles?: Divide the mass in grams by the molar mass in grams per mole, which you get by summing the atomic masses in the formula.
What is the difference between molarity and molality?: Molarity is per litre of solution and varies with temperature. Molality is per kilogram of solvent and does not, which matters for freezing and boiling point work.
Should I add solvent to the mark or measure it first?: Dissolve in a smaller volume then make up to the mark. Adding solute to a full litre gives more than a litre of solution and a concentration below target.