By Michael Merry
Updated Aug 30, 2022

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Understanding Concentration in Chemistry

In chemical work, knowing how much of a substance is dissolved in a given volume of solution is essential. This measurement is known as concentration, and molarity is the most widely used unit for expressing it. Molarity aligns perfectly with stoichiometric calculations because reactants combine in whole‑number mole ratios. For instance, the reaction 2 H₂ + O₂ → 2 H₂O involves 2 moles of hydrogen gas reacting with 1 mole of oxygen gas to yield 2 moles of water.

The Mole: A Counting Unit for Atoms and Molecules

A mole is defined as the amount of a substance that contains exactly 6.022 × 10²³ elementary entities—atoms, molecules, ions, or other particles. This value, known as Avogadro’s number, was agreed upon internationally based on the number of atoms in 12 g of the carbon‑12 isotope. Using this counting unit simplifies mass calculations: 1 mol of oxygen weighs 16.00 g, 1 mol of water 18.02 g, and 1 mol of carbon dioxide 44.01 g.

What Is Molarity?

Molarity, denoted by the symbol M, is the number of moles of a solute dissolved in one liter of solution. It differs from molality, which expresses moles of solute per kilogram of solvent. Molarity is preferred in laboratory settings because it directly relates to the volume of solution used in reactions.

Calculating Molarity: A Step‑by‑Step Example

Suppose you need the molarity of a solution containing 100 g of sodium chloride (NaCl) in 2.5 L of solution.

Step 1: Determine the formula weight of NaCl.
Na (22.99 g/mol) + Cl (35.45 g/mol) = 58.44 g/mol.

Step 2: Find the number of moles of NaCl.
100 g ÷ 58.44 g/mol = 1.71 mol.

Step 3: Calculate molarity.
1.71 mol ÷ 2.5 L = 0.684 M.

Preparing a Solution of a Desired Molarity

How much sodium sulfate (Na₂SO₄) is needed to make 250 mL of a 0.5 M solution?

Step 1: Compute required moles.
0.25 L × 0.5 mol/L = 0.125 mol.

Step 2: Calculate formula weight of Na₂SO₄.
2 Na (22.99 g/mol × 2) + S (32.07 g/mol) + 4 O (16.00 g/mol × 4) = 142.1 g/mol.

Step 3: Determine mass of Na₂SO₄.
0.125 mol × 142.1 g/mol = 17.76 g.

These straightforward calculations illustrate how molarity enables precise preparation and analysis of chemical solutions.