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Whether you’re conducting a lab experiment or tackling an exam, calculating molarity is a foundational skill in chemistry. Molarity quantifies how concentrated a solution is, expressed as the number of moles of solute per liter of solution. The calculation requires only a simple formula and a few key pieces of data.

Understanding the Formula

In its most basic form, molarity (M) is calculated by dividing the number of moles of solute by the volume of the solution in liters:

M = moles of solute ÷ liters of solution

This relationship is often written as “mol L⁻¹” or “moles per liter.”

Figuring Out Moles

To determine the moles of a solute, you need the mass of the solute, its chemical formula, and the atomic weights of its constituent elements (obtainable from the periodic table). Sum the atomic weights to find the molar mass, then divide the mass of the solute by that molar mass.

Measuring the Solution

The volume of the solution must be expressed in liters for the molarity formula. If the volume is given in milliliters, divide by 1,000 to convert to liters.

Putting It Together

Example: Dissolve 20 g of NaOH in 500 mL of water.

1. Convert volume: 500 mL ÷ 1,000 = 0.500 L.
2. Determine molar mass of NaOH: Na (23 g) + O (16 g) + H (1 g) = 40 g mol⁻¹.
3. Calculate moles: 20 g ÷ 40 g mol⁻¹ = 0.500 mol.
4. Compute molarity: 0.500 mol ÷ 0.500 L = 1.0 M.

Thus, the resulting solution has a molarity of 1 mol L⁻¹.

Going Further

Once you master the basic formula, you can rearrange it to solve for any unknown—whether it’s the amount of solute, the number of moles, or the volume of solution—using algebraic manipulation. This flexibility is essential for stoichiometric calculations, titration analyses, and preparing solutions of desired concentrations.