Concentration of Solution
Concentration tells us how much of a solute (substance) is dissolved in a solution. It is a way to measure how “strong” or “weak” 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. [1-4]
Concentration is very important in chemistry because it affects how substances react with each other. A solution that is too weak or too strong can change the outcome of a chemical reaction.
How to Calculate the Concentration of Solution [1-4]
The concentration is measured using several formulas and units, such as:
1. 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 % = (Mass of solute / Mass of solution) x 100
Where:
– Mass of solute is the weight of the substance being dissolved (e.g., salt or sugar).
– Mass of solution is the total weight, which includes 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.
2. Molarity
Molarity (M) is defined as the number of moles of solute present in one liter of solution. It is used to describe how concentrated a solution is, particularly when dealing with liquids.
Formula
The formula for calculating molarity is:
M = Moles of solute / Volume of solution in liters
Where:
– Moles of solute is the amount of solute in moles. A mole represents 6.022 x 1023 particles of the solute.
– Volume of solution in liters refers to the total volume of the solution, not just the solvent.
Units
The unit of molarity is moles per liter, often abbreviated as ML-1.
3. Molality
Molality (m) is defined as the number of moles of solute dissolved in one kilogram of solvent. It describes the number of particles of the solute present in every kilogram of solvent and is often used to study properties like boiling point elevation and freezing point depression.
Formula
The formula for calculating molality is:
m = Moles of solute / Mass of solvent in kilograms
Where:
– Moles of solute refers to the amount of solute measured in moles.
– Mass of solvent in kilograms is the weight of the solvent used to dissolve the solute.
Units
The unit of molality is moles per kilogram (mol/kg), often written as m.
4. Parts Per Million and Parts Per Billion
Parts Per Million (ppm) 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.
Parts Per Billion (ppb) represents the number of parts of solute per one billion parts of the solution. This unit is used for even smaller concentrations than ppm.
Formula
The general formulas for ppm and ppb are:
ppm = (Mass of solute / Mass of solution) x 106
ppb = (Mass of solute / Mass of solution) x 109
Where:
– Mass of solute and mass of solution can be in any consistent units (grams, kilograms, etc.), but the final result is a dimensionless number.
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).
Units
Ppm is expressed as milligrams of solute per liter of solution (mg/L) in aqueous solutions where water is the solvent.
Ppb is expressed as micrograms of solute per liter of solution (μg/L) for water-based solutions.
5. Normality
Normality (N) 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:
N = (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.