Concentration of Solution
Table of Contents
Concentration refers to the quantity of a substance (solute) dissolved in a liquid (solvent) to form a solution. It measures how “concentrated” or “dilute” the solution is. For example, if you add one teaspoon of sugar to a glass of water, the solution is not very concentrated. But if you add five teaspoons of sugar to the same glass of water, the solution becomes much sweeter and more concentrated. Concentration is very important in chemistry because it affects how substances react with each other. [1-4]
Units of Concentration [1-4]
The concentration of a solution is measured by using several formulas and units, such as:
1. Molarity
Molarity is defined as the number of moles of the solute present in one liter of solution.
Formula
The formula for calculating molarity is:
Molarity (M) = (Number of moles of solute)/(Volume of solution in liters)
Units
The unit of molarity is moles per liter (mol/L), often abbreviated as ML-1.
2. Molality
Molality is defined as the number of moles of solute dissolved in one kilogram of solvent. It is used to study colligative properties such as vapor pressure lowering, boiling point elevation, and freezing point depression.
Formula
The formula for calculating molality is:
Molality (m) = (Number of moles of solute)/(Mass of solvent in kilograms)
Units
The unit of molality is moles per kilogram (mol/kg), often written as Mkg-1.
3. Mass Percent
Mass percent is defined as the ratio of the mass of the solute to the total mass of the solution multiplied by 100. This gives the solute’s percentage by weight in the solution.
Formula
The formula for calculating mass percent is:
Mass percent = (Mass of solute/ Mass of solution) x 100
The mass of the solution includes masses of both the solute and the solvent.
Units
Mass percent is expressed as a percentage (%). Both the solute and solution masses must be in the same unit (e.g., grams or kilograms) to ensure consistency.
4. Parts Per Million and Parts Per Billion
Parts Per Million represent the number of parts of solute per one million parts of the solution. If one part of solute is present in one million parts of a solution, the concentration is 1 ppm.
On the other hand, parts per billion represents the number of parts of the solute per one billion parts of the solution. This unit is used for even smaller concentrations than ppm.
Formula
The general formulas for parts per million (ppm) and parts per billion (ppb) are:
ppm = (Mass of solute/ Mass of solution) x 106
ppb = (Mass of solute/ Mass of solution) x 109
If the solution is liquid, its mass is often approximated by its volume in liters (assuming its density is close to that of water, 1 g/mL).
5. Normality
Normality is defined as the number of equivalents of solute dissolved in one liter of solution. An equivalent is the amount of a substance that reacts with or supplies one mole of hydrogen ions (H+) in an acid-base reaction or one mole of electrons in a redox reaction.
Formula
The formula for normality is:
Normality (N) = (Number of equivalents of solute)/(Volume of solution in liters)
To calculate the number of equivalents:
Equivalents of solute = (Mass of solute/ Equivalent weight of solute)
Where equivalent weight is the molar mass divided by the valency (number of H+, OH−, or electrons involved per molecule of the substance in the reaction).
Units
The unit of normality is equivalents per liter (eq/L), typically written as N.