Mastering Molarity: A Step‑by‑Step Guide to Calculating Initial Concentrations

By Contributing Writer • Updated March 24, 2022

Calculating the initial concentration of a solution—commonly called molarity—is a foundational skill in chemistry and biochemistry. Molarity expresses how many moles of solute are present per liter of solution.

Step 1: Convert the Mass of Solute to Moles

Weigh the solute in grams, then divide by its molar mass. For example, sodium hydroxide (NaOH) has a molar mass of 40.0 g mol⁻¹. If you dissolve 20.0 g of NaOH:

mol NaOH = 20.0 g × (1 mol / 40.0 g) = 0.50 mol

Step 2: Determine the Volume of Solvent in Liters

Measure the solvent volume. If the measurement is in milliliters, convert to liters (1 L = 1000 mL). For instance, 500 mL of solvent equals:

500 mL × (1 L / 1000 mL) = 0.500 L

Step 3: Calculate Molarity

Divide the moles of solute by the solvent volume in liters:

M = 0.50 mol / 0.500 L = 1.0 M NaOH

Thus, the solution has a molarity of 1 M. Increasing the solvent volume would lower the molarity, while reducing it would raise the concentration.

TL;DR (Too Long; Didn’t Read)

Keep track of units throughout the calculation—convert grams to moles and milliliters to liters—to avoid conversion errors when determining molarity.