By Kevin Beck
Updated August 30, 2022
In chemistry, concentration is expressed in terms of moles rather than mass or volume alone. A mole represents exactly 6.022 × 10²³ particles of a substance, the same number of atoms that constitute 12 g of the most common isotope of carbon (C‑12). Knowing the molar mass of each element allows chemists to convert between mass and moles with precision.
One mole of any substance contains 6.022 × 10²³ discrete entities. For example, 1 mol of water (H₂O) weighs 18.015 g because the molar mass of oxygen (15.999 g mol⁻¹) plus twice the molar mass of hydrogen (2 × 1.008 g mol⁻¹) equals 18.015 g.
Molarity (M) is the number of moles of solute per liter of solution. It is commonly used to describe the concentration of solutions in laboratory settings. For instance, a 1 M solution of calcium chloride (CaCl₂) dissociates according to:
CaCl₂(aq) ⇌ Ca²⁺(aq) + 2Cl⁻(aq)This reaction produces 1 mol of Ca²⁺ ions and 2 mol of Cl⁻ ions per mole of CaCl₂.
Normality (N) measures concentration in gram equivalents per liter. The formula is:
N = M × nwhere n is the number of equivalents per mole—essentially the number of charge units a species can donate or accept in a reaction. For CaCl₂, n = 2 (one Ca²⁺ and two Cl⁻ ions), so a 1 M solution corresponds to 2 N.
To convert normality to molarity, divide by the equivalence factor:
M = N / n. Conversely, multiply molarity by n to obtain normality. For example, 0.5 N NaOH (a strong base that reacts with a single proton per molecule) has a molarity of 0.5 M because n = 1.
For routine calculations involving acids and bases, online molarity and normality calculators can streamline the process. These tools allow you to input molar mass, desired concentration, and equivalence factors to obtain accurate results.
See the Resources section for a convenient calculator that handles a wide range of acid–base combinations.
