Summary
Highlights
Concentration of solutions tells us how much solute is present per given amount of solution or solvent. Solute is the dissolved substance, and solvent is the dissolving medium. Concentration is expressed as the quantity of solute over the quantity of solution or solvent. The numerator is always the solute, while the denominator can be either the solution or the solvent.
Percent by mass is the ratio of mass of solute divided by the mass of solution, usually expressed in grams, multiplied by 100. The mass of the solution is the sum of the mass of the solute and the mass of the solvent. An example problem demonstrates calculating percent by mass when given the mass of solute and the mass of solution.
Percent by volume uses the volume of solute divided by the volume of solution, typically in milliliters, multiplied by 100. Unlike mass, volume is not additive. To prepare a solution by volume, you add the solvent until the desired total volume is reached, not by simply adding predefined volumes of solute and solvent. An example calculation shows how to find the volume of alcohol in wine given its percent by volume.
Percent by mass-volume combines mass and volume measurements. It's the ratio of the mass of solute (in grams) to the volume of the solution (in milliliters), multiplied by 100. This is useful when dissolving a solid solute in a liquid solvent. An example problem illustrates how to find the mass of solute in a given mass-volume solution.
Mole fraction expresses the relative amounts of substances in a mixture as a ratio of moles of one substance to the total number of moles of all substances. To calculate moles, divide the mass of a substance by its molar mass. The sum of all mole fractions in a solution always equals one. A detailed example shows calculating mole fractions for sulfuric acid in an aqueous solution.
Molality (denoted by 'm') is the ratio of the number of moles of solute to one kilogram of solvent. This is distinct from molarity because it uses the mass of the solvent, not the solution. Moles are calculated from mass and molecular mass. An example demonstrates how to find the molal concentration, including converting solvent volume to mass using density.
Molarity (denoted by 'M') is similar to molality but represents the moles of solute per liter of solution. This unit is frequently used in stoichiometry. An example problem illustrates calculating molarity, requiring conversion of the solution's volume from milliliters to liters.
Parts per million (ppm) expresses the number of parts of solute per one million parts of solution. It can be calculated as grams of solute per million grams of solution, or milligrams per liter, or milligrams per kilogram, depending on the specific situation. It's a method for expressing very dilute concentrations.