A solution is a homogeneous mixture where one substance is completely dissolved into another. This mixture is composed of two primary parts: the solute and the solvent. The solute is the substance present in the smaller amount (e.g., salt or sugar) that is dissolved by the solvent, which is the substance present in the larger amount (typically water).
The concentration of a solution is a measure that describes the amount of solute relative to the total amount of solution. This ratio allows scientists and manufacturers to precisely control the properties of a mixture, whether preparing a drug, formulating a cleaning product, or analyzing a water sample.
Basic Expression: Mass and Volume Percentages
One of the most straightforward ways to express concentration is through a percentage, which tells you how many parts of solute are present per 100 parts of the total solution. These percentage expressions are often used for common products and are easily understood without specialized chemistry knowledge.
Mass percent, or mass/mass percent \((\% \mathrm{m/m})\), is calculated by dividing the mass of the solute by the total mass of the solution and multiplying by 100. For instance, a 10% salt solution by mass means that 10 grams of salt are dissolved in 90 grams of water, creating 100 grams of total solution. Volume percent \((\% \mathrm{v/v})\) is used when both the solute and solvent are liquids, such as alcohol in water, using the ratio of solute volume to total solution volume.
Another common unit is mass/volume percent \((\% \mathrm{m/v})\), frequently used in biological and medical contexts. This measure expresses the mass of the solute (in grams) dissolved in a final volume of solution (in milliliters), multiplied by 100. While convenient for everyday use, percentage concentrations do not account for the number of particles present, which limits their utility in complex chemical reactions.
Molar Concentration
For scientists, Molar Concentration, or Molarity (\(M\)), is the standard unit for expressing concentration because it provides a measure related to the actual number of particles in the solution. Molarity is defined as the number of moles of solute dissolved per liter of the total solution volume (\(M = \text{moles of solute} / \text{liters of solution}\)).
A mole is an aggregate of exactly \(6.022 \times 10^{23}\) particles, a number known as Avogadro’s number. This number ensures that one mole of any substance has a mass in grams numerically equal to its molecular weight. For example, water (\(H_2O\)) has a molecular weight of approximately 18.02 units, meaning one mole of water weighs 18.02 grams.
Molarity allows for precise control of stoichiometry, the measurement of the proportions of reactants and products. By knowing the Molarity, a chemist can calculate exactly how much volume of a solution is needed to provide a specific number of reactant particles for a reaction. A solution labeled “2 M” means there are 2 moles of the solute dissolved in every 1 liter of the solution, allowing for much more accurate and reproducible experiments compared to mass or volume percentages.
Trace Concentrations: Parts Per Million and Billion
When measuring extremely small amounts of a substance, such as contaminants in air or water, concentration is expressed using “parts-per” notation. The most common trace concentration units are Parts Per Million (ppm) and Parts Per Billion (ppb).
Parts Per Million (\(ppm\)) represents one unit of solute in one million units of the total solution. The concentration of carbon dioxide in the Earth’s atmosphere, for example, is typically measured in ppm.
Parts Per Billion (\(ppb\)) is a unit a thousand times smaller than ppm, representing one part of solute in one billion parts of the solution. One ppb is analogous to a single second in approximately 32 years. These units are frequently used by regulatory bodies like the Environmental Protection Agency to set maximum contaminant levels for drinking water, such as for lead or certain pesticides.
Measuring these trace amounts is important for health and environmental monitoring. Although the concentrations are tiny, many toxic substances, like heavy metals or certain pollutants, can have significant biological effects even at ppb levels. The ability to accurately measure these minute concentrations ensures public safety and allows scientists to track the long-term changes of trace gases in the atmosphere.