There are many simple yet essential topics in chemistry that are both easy to understand and exceptionally important. And concentration in chemistry is one of them. Along with being one of the most basic concepts in chemistry, many aspects of chemistry pretty much depend on it.
The concentration of a solution refers to the amount of solute being dissolved in a specific amount of solvent.
But how is concentration used in chemistry? Or why is concentration so important for understanding pretty much every branch of chemistry? Here’s the answer for you.
In this article, you’ll learn more about what concentration in chemistry is and what the units of concentration are. You’ll also find a few reasons why concentration is so important in chemistry.
What Is Concentration? – Definition
Along with defining concentration let’s discuss a few terms that will help you understand the concept.
The concentration of a solution measures the amount of solute that has been dissolved in a solvent to create a solution.
A solute is a substance that is being dissolved.
A solvent is a substance in which a solute is dissolved.
A concentrated solution is a solution in which a large amount of dissolved solute is present. To concentrate a solution, we add the original solute to the solution. Alternatively, we can reduce the amount of solvent. For example, heating a solution can also make it more concentrated as the solvent is likely to evaporate during the process.
A dilute solution is a solution in which a small amount of dissolved solute is present. To dilute a solution, we add the original solvent to the solution.
So, when the solute-to-solvent ratio is increased, the solution becomes more concentrated. Similarly, when the solute-to-solvent ratio is decreased, the solution becomes more dilute.
Read more about the concentration of solutions in this chapter on Chem LibreTexts.
Quantitative Units of Concentration and Corresponding Equations in Chemistry
While numbers indicate how much of a solute has been dissolved in a solvent, units are essential for determining the values.
The quantitative units of concentration are molarity, molality, normality, mass concentration, volume concentration, number concentration, mass percent, volume percent, mass/volume percent, mole fraction, mass fraction, mole ratio, mass ratio, ppm, and ppb.
Below are the units of concentration and corresponding formulas to use when solving chemistry problems or preparing solutions in a lab.
Molarity (M) – mol/L
Molar concentration, also referred to as molarity or amount concentration, is one of the quantitative units of concentration. Molarity can be defined as the number of moles of solute per liter of solution.
The unit of molarity is mol/L and molarity can be calculated using the following equation:
- Molarity (M) = moles (solute)liters (solution)
Khan Academy explains how to calculate the molar concentration (molarity) of a solution, so make sure you check it out.
Molality (m) – mol/kg
While molarity is typically used when calculating the concentration of a solution, it doesn’t really work if the temperature is changing. This is because the volume of a given solution changes as the temperature increases or decreases. In this case, we use molality, which is defined as the number of moles of a solute divided by the kilograms of a solvent.
The unit of molality is mol/kg and molality can be calculated using the following equation:
- Molality (m) = moles (solute)kilograms (solvent)
Check out this video to better understand the differences between molarity (M) and molality (m).
Normality (N) – eq/L
Normality is yet another expression in chemistry that is used to measure the concentration of a solution. Normality is typically used for measuring the concentration of reactive species in the case of titration reactions. Normality is defined as the number of gram equivalents of a solute divided by the liters of a solution.
The unit of normality is eq/L and normality can be calculated using the following equation:
- Normality (N) = number of gram equivalents (solute)liters (solution)
- Number of Gram Equivalents = weight (solute)equivalent weight (solute)
- Equivalent Weight = molecular weightvalence
The Organic Chemistry Tutor explains how to calculate normality and equivalent weight for acid-base reactions.
Mass Concentration (ρ) – kg/m3
Mass concentration is defined as the ratio of the mass of a solute to the volume of a solution.
The unit of mass concentration is kg m-3 and mass concentration can be calculated using the following equation:
- Mass Concentration (ρi) = mass (solute)volume (solution)
Volume Concentration (σ) – no unit
Similarly to mass concentration, volume concentration is the ratio of the volume of a solute to the volume of a solution.
Volume concentration doesn’t have a unit and it can be calculated using the following equation:
- Volume Concentration (σ) = volume (solute)volume (solution)
Number Concentration – 1/m3
Number concentration is yet another quantity that’s used to calculate the concentration of a solution. Number concentration can be defined as the number of entities (atoms, molecules) of a solute divided by the volume of the solution.
The unit of number concentration is 1/m3 and number concentration can be calculated using the following equation:
- Number Concentration = number of entities (solute)volume (solution)
Mass Percent – %
Mass percent of a solution is the ratio of the mass of a solute to the mass of a solution multiplied by 100%.
The unit of mass percent is % and mass percent can be calculated using the following equation:
- Mass Percent = mass (solute)mass (solution)100%
Volume Percent – %
Similarly to mass percent, the volume percent of a solution is the ratio of the volume of a solute to the volume of a solution multiplied by 100%.
The unit of volume percent is % and volume percent can be calculated using the following equation:
- Volume Percent = volume (solute)volume (solution)100%
Mass/Volume Percent – %
When calculating the concentration of a solution, you may also be asked to determine the mass/volume percentage concentration. Mass/volume percent is defined as the mass of a solute divided by the volume of a solution and multiplied by 100%.
The unit of mass/volume percent is % and mass/volume percent can be calculated using the following equation:
- Mass/Volume Percent = mass in grams (solute)volume in mL (solution)100%
Read more about the percentage concentration of solutions in this chapter on Chemistry LibreTexts.
Mole Fraction – no unit
Mole fraction is a quantity used when there are multiple components in a solution. The mole fraction of a solution is the ratio of the number of moles of a specific component to the total number of moles of all components present in a given solution.
So, if there are A, B, and C components in a solution and we’re asked to find the mole fraction of component A, we should use the following equation:
- Mole Fraction (XA) = moles (A)moles (A) + moles (B) + moles (C)
Conquer Chemistry shares a few examples to explain how to calculate mole fraction, so make sure you check it out.
Mass Fraction – no unit
Mass fraction, also referred to as weight fraction, is pretty similar to mole fraction. The only difference is that we use the masses of the constituents instead of moles. So, the mass fraction of a solution is the ratio of the mass of a specific constituent to the total mass of a given solution.
If our solution consists of A, B, and C components, the mass fraction of component A can be calculated using the following equation:
- Mass Fraction (wt/wt) = mass (A)mass (A) + mass (B) + mass (C)
To master the conversions between mass and mole fractions, check out this video tutorial by LearnChemE.
Mole Ratio – no unit
While many students often confuse mole ratio with mole fraction, these are two different quantities. The mole ratio is the ratio of the moles of a compound to the moles of another compound in a solution.
For example, you may be asked to prepare a solution that has a 1:4 molar ratio of pentane to hexane.
When calculating the molar ratio, you shouldn’t forget to balance the chemical equation first. For detailed instructions on how to solve mole ratio problems, check out this video by Tyler DeWitt or this one by The Organic Chemistry Tutor.
Mass Ratio – no unit
According to the Law of Conservation of Mass, matter is neither created nor destroyed in a chemical reaction. Although components can be transformed into different compounds, their initial mass doesn’t change.
To calculate the mass ratio in a compound AB, we divide the mass of A by the mass of B. Chem LibreTexts explains mass ratio calculations by providing a simple example.
PPM (Parts per Million)
When we’re dealing with extremely dilute solutions, we often use the quantity called ppm, or parts per million. So if ppm=1, there is one part of a solute per 1,000,000 (106) parts of a solution. ppm can be calculated using the following equation:
- Parts per Million = ppm = grams (solute)grams (solution)x106
PPB (Parts per Billion)
When we have even more dilute solutions than mentioned above, we calculate the solution concentration in parts per billion, or ppb. So, if ppb=1, there is one part of a solute per 1,000,000,000 (109) of a solution. ppm can be calculated using the following equation:
- Parts per Billion = ppb = grams (solute)grams (solution)x109
If you’re getting confused when calculating solution concentrations in ppm or ppb, check out this quick video guide.
Why Is the Concept of Concentration So Important in Chemistry?
While chemistry textbooks include hundreds if not thousands of concentration problems to solve, these are not just random numbers and sentences crafted for educational purposes. These problems are real-life experiments that are performed in labs on a daily basis. So, why is the concept of concentration so important in chemistry?
Concentration is important because it’s key to preparing solutions. It also allows scientists to run chemical experiments and reactions. Besides, concentration is essential for quantitative analysis in analytical chemistry and synthesis reactions in organic chemistry. Last but not least, concentration is fundamental to assessing blood and body fluid tests in medicine.
When preparing solutions, we use basic calculations and different concentration units to reach the desired concentration of a solution. Besides, when diluting or making solutions more concentrated, we use these calculations once again.
Whether we’re conducting basic chemical experiments or more advanced research, the concept of concentration allows us to achieve the goal.
When it comes to quantitative analysis in analytical chemistry, chemists prepare a broad variety of solutions, ranging from extremely dilute to very concentrated ones. And the same applies to the organic synthesis reactions. Without the concept of concentration, scientists wouldn’t be able to perform such experiments.
And finally, concentration calculations are used when analyzing blood and body fluid tests. The concept of concentration is essential for determining the amount of a substance of interest present in a given blood or body fluid sample.