Units of Chemical Concentration
Concentration is a term used to describe the "strength" of a solution. Even with no scientific background, most people understand the concept of terms concentrated and dilute in terms of solutions. In many scientific fields, including chemistry, it is necessary and useful to quantitate the amount of concentration in a solution - that is, how much "stuff" is dissolved in a container of liquid.
A solution is a homogenous mixture of one or more solutes in a solvent. For example, dissolving salt or sugar in a glass of water creates a homogeneous solution. That is, the salt or sugar and the water are so well mixed that, even with a microscope, one cannot see the individual particles in the mixture. A solution in which the individual components can be seen is called a heterogeneous solution, such as Italian salad dressing or trail mix. We can standardize the way we describe a solution's concentration by applying numbers and units to it, allowing us to compare two different solutions based on their concentrations.
Below are some common Units of Concentration used in chemistry and other fields.
Percent by Volume (%v/v)
Percent by volume (%v/v) is often used for more concentrated solutions. The most common use of this concentration is percent alcohol by volume (%ABV), written on many beer labels and restaurant menus. The calculation is simply the volume of solute (e.g., alcohol) divided by the volume of the solution (e.g., beer), and multiplied by 100%. Unless otherwise noted, the volumes used in the calculation are in the same units (e.g., mL/mL or liter/liter).
For example, a beer with 5.5% ABV means that in 100 mL of beer, there are 5.5 mL of alcohol present. Put another way, a 12 oz bottle of beer would contain 0.66 oz of alcohol.
Percent by Mass (%w/w)
Percent by mass (%w/w) is used for more concentrated solutions, including solid mixtures. Like percent by volume above, it is simply the ratio of solute to solution, but this time the units are in mass (e.g., mg/mg or grams/grams). For example, a 100. g solution of saline containing 2.5 grams of sodium chloride would have a concentration of 2.5 % by mass.
The values of percent by mass and percent by volume of the same solution will usually not be the same value because of the densities of materials involved. For example, a beer with 5.5% ABV will have a lower percent by mass value because alcohol (ethanol) is less dense than water. A beer with 5.5% ABV will be approximately 4.3% by mass.
Percent Mass by Volume (%w/v)
Percent mass by volume (%w/v) is sometimes used for dilute concentrations, and is very similar to parts per million in units. In this case, the mass of solute in grams is divided by the volume of solution (standard is 100 mL) and multiplied by 100%. For example, commercial vinegar is typically made of 5.5 grams of acetic acid per 100 mL of vinegar, giving a 5.5% w/v concentration
Parts Per Million (ppm)
Parts-per-million (ppm) is used to describe very dilute solution concentrations, such as salts or metal in drinking water. Commonly, the units for ppm are given as milligrams solute per Liter of solution (mg/L). This is possible because 1 liter (1 L) of water has a mass of 1000 grams, or 1 kilogram (1 kg). A milligram (mg) is 1/1000th of a gram (or 0.001 g), and a kilogram has 1000 grams. Therefore, 1 mg of a substance in 1 liter of water would be 0.001 divided by 1000, which is 1/1,000,000th or 1 part in 1 million.
Parts Per Billion (ppb)
Parts-per-billion (ppb) is used to describe very dilute solution concentrations, such as toxic metals in drinking water. Commonly, the units for ppb are given as micrograms solute per Liter of solution (μg/L). This is possible because 1 liter (1 L) of water has a mass of 1000 grams, or 1 kilogram (1 kg). A microgram (μg) is 1/1,000,000th of a gram (or 0.000001 g), and a kilogram has 1000 grams. Therefore, 1 μg of a substance in 1 liter of water would be 0.000001 divided by 1000, which is 1/1,000,000,000th or 1 part in 1 billion.
Molarity (M) is most often used in chemistry, and it is defined as the moles of solute (mol) per Liter of solution (mol/L). A bottle labeled 1.50 M NaCl is read as "1.50 molar sodium chloride", meaning there are 1.50 moles of sodium chloride in 1 Liter of this solution.
Normality (N) is a bit antiquated but is still used in chemistry, medicine, and other fields. It is defined as the number of molar equivalents per Liter of solution. For example, if hydrochloric acid (HCl) is dissolved into water, the compound dissociates into its constituent ions (H+ cations and Cl- anions), so for every 1 mol of HCl, you get 1 mol H+ and 1 mol of Cl- ions. In this case, 1 M HCl (mol/L) is equal to 1 N NaCl.
Now, in the case of a sulfuric acid (H2SO4), it dissociates in water to give 2 mol H+ cations and 1 mol of SO42- anions. Here there are 2 molar equivalents of H+ per Liter of solution. Therefore 1 M H2SO4 is 2 N H2SO4 since there are 2 H+ for every one molecule of acid (H2SO4).
Notes & References
- The density of water is approximately 1.00 g/mL, whereas the density of ethanol is approximately 0.789 g/mL.
- The density of water is approximately 1.00 gram per milliliter (g/mL). A liter contains 1000 milliliters and therefore has a mass of 1000 grams.