Molarity

Before you rely on it

What to check

Note that it is per litre of final solution, not per litre of solvent added.

The common error

Where people go wrong with molarity

Adding solute to a litre of water and calling the result one molar. For a concentrated solution the volume increases, and the actual molarity comes out below the target.

Worked through

Molarity in plain numbers

One mole of table salt weighs 58.44 grams. Dissolve that amount in enough water to make one liter of solution and you have a 1.00 molar solution, written 1 M. Dissolve 5.844 grams and make it up to 250 milliliters: that is 0.1 mole in 0.25 liters, or 0.4 M. To make 100 milliliters of 0.1 M from a 1 M stock, use C₁V₁ = C₂V₂: (1 M)(V₁) = (0.1 M)(100 mL), so V₁ = 10 mL of stock, then add solvent up to 100 mL. Molarity is moles per liter of solution, not per liter of solvent, so volume is measured after mixing.

Easily confused

Molarity and its neighbours

Molarity is not molality, and it is not percent concentration. Molality is moles of solute per kilogram of solvent, and it does not change with temperature. Molarity is per liter of solution, and it changes slightly because volume changes with temperature. Percent by mass or volume uses different bases. Normality, an older unit, depends on the reaction. Parts per million is used for very dilute solutions. Chemists pick the unit that fits the work, and conversions need the molar mass and, often, the density.

In real decisions

Where molarity turns up

Students and lab workers prepare solutions, dilute stock solutions and calculate reagent quantities in titrations and reactions. Pharmacists and biologists prepare buffers and media. Environmental chemists express concentrations of pollutants. Industry uses molarity in process control; a molarity calculator converts among grams, moles, volume and concentration, saving arithmetic and avoiding errors when preparing a solution.

Know the edges

Where molarity stops being reliable

It assumes ideal mixing, and very concentrated solutions deviate. Temperature changes volume, so it applies at the stated temperature. Purity and hydration of the solute affect the mass needed. Always follow laboratory safety rules and protocols. This page is educational. For actual preparation of chemicals, use verified procedures and safety data from the supplier.

Quick answers

What people ask about molarity

Why is volume measured after mixing? Molarity is moles per liter of solution, and dissolving a solute can change the total volume. Fill a volumetric flask to the mark after the solute dissolves.

What is the difference between molarity and molality? Molarity uses liters of solution, molality uses kilograms of solvent. Molality does not change with temperature, so it is preferred in some work.

How do I dilute a solution? Use C₁V₁ = C₂V₂. To make 250 mL of 0.2 M from a 1 M stock, take 50 mL of stock and add solvent to 250 mL. Follow the safety rules for the chemicals.

Does temperature affect molarity? Slightly. Volume changes with temperature, so the concentration shifts. Prepare and use solutions at the temperature noted.