Preparing solutions is one of the most fundamental skills in any chemistry lab. Whether you are a first-year student making your first sodium chloride solution or an experienced researcher preparing a complex buffer, getting the concentration right matters. A small error in concentration can throw off an entire experiment, and in some cases, like pharmaceutical preparations, it can have serious consequences.
Types of Concentration
There are several ways to express the concentration of a solution. Molarity (M) is the most common in chemistry and is defined as moles of solute per liter of solution. Molality (m) is moles of solute per kilogram of solvent — the distinction matters when temperature changes affect the volume of the solution. Mass percent is the mass of solute divided by the total mass of solution, multiplied by 100. Parts per million (ppm) is used for very dilute solutions. Our Solution Concentration Calculator can convert between molarity, molality, and mass percent.
Making a Molar Solution
Let us say you need 250 mL of a 0.5 M NaCl solution. First, calculate how many moles you need: 0.5 mol/L x 0.250 L = 0.125 mol. Then convert to grams using the molar mass: 0.125 mol x 58.44 g/mol = 7.305 g. Weigh out 7.305 g of NaCl, transfer it to a 250 mL volumetric flask, add some distilled water, swirl to dissolve, then fill to the calibration mark. Never add solute directly to a full volumetric flask — dissolve first, then bring to volume.
Dilution
Dilution is the process of reducing the concentration of a solution by adding more solvent. The relationship C1V1 = C2V2 is all you need to remember. If you have a 6 M HCl stock solution and need 500 mL of 1 M HCl, you solve for V1: V1 = C2V2/C1 = (1 x 500)/6 = 83.3 mL. So you measure 83.3 mL of the concentrated acid and dilute to 500 mL. Always add acid to water, never the reverse, to avoid dangerous splashing. The Dilution Calculator handles these calculations for you.
Safety note: Concentrated acids and bases are corrosive. Always wear gloves, goggles, and a lab coat. Work in a fume hood when handling volatile or strong-smelling chemicals. Label every solution you prepare with the chemical name, concentration, date, and your initials.