An introduction to Concentration of Solution
Everyone talks about the concentration of solutions. They may also talk about the concentration of coffee or tea. Everyone has a particular view of what is meant by the concentration of a solution. You must have noticed that whenever you make coffee, if you add a lot of powder, you will end up with a concentrated drink, whereas if you add little, it will result in a dilute solution. Therefore, it is essential that you understand what the concentration of the solution is. In this chapter, we will learn about what is meant by the concentration of a solution; we will also see how to find the concentration of a solution and the different methods of expressing the concentration of the solution.
What is Concentration of a Solution?
In an aqueous solution, two parts exist, namely solute and solvent. They are the two basic solution concentration terms that you need to know. We always need to keep an account of the amount of solute in the solution. In chemistry, we define the concentration of solution as the amount of solute in a solvent. When a solution has more solute in it, we call it a concentrated solution. Whereas when the solution has more solvent in it, we call it a dilute solution.
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Now that you understand the concept of what is the concentration of solution let's move on to the different methods of expressing concentration.
Methods of Expressing the concentration of Solution
There are various methods of expressing the concentration of a solution. You will usually see Chemists working with the number of moles. Pharmacists will use percentage concentrations instead of the number of moles. Hence it is important to understand all the methods of expressing the concentration of solutions.
The concentration of the solution formula is given as follows.
Concentration of solution = \[\frac{\text{Weight of the solute in gram}}{\text{volume in Litres}}\]
We will also see other methods on how to calculate the concentration of a solution based on the different methods of expressing concentrations.
Concentration in Parts per Million
It is expressed in terms of weight. The formula for parts per million is given as follows:
ppm(A)= \[\frac{\textrm{Mass of A}}{\textrm{Total mass of the solution}}\]x10\[^{6}\]
Mass Percentage (w/w)
It is expressed in terms of mass percentage of solute to the solution. The formula for mass percentage is given as follows.
Mass percentage of A = \[\frac{\text{Mass of component A}}{\text{Total mass of the soution}}\]x100
e.g. CH3COOH 33% w/w, and H2SO4 98.0% w/w.
Volume Percentage (V/V)
It is expressed in terms of the volume percentage of solute to the solvent. The formula for volume percentage is given as follows.
Volume percentage of A = \[\frac{\text{Volume of component A}}{\text{Total volume of the solution}}\]x100
Mass by Volume Percentage (w/V)
Percentage weight in volume expresses the number of grams of solute in 100 ml of product.
e.g. BaCl2 solution 10% w/v, and H2O2 solution 5-7% w/v.
Molarity (M)
It is the number of moles of solute contained in 1000 ml of solution. It is a commonly used method for expressing concentrations.
Molarity = \[\frac{\text{Mass of solute}}{\text{volume of solution in litres}}\]
Molality (m)
The molality is expressed as the number of moles of a solute contained in 1000 gm of a solvent. The formula for molality is given as follows.
Molality (m) = \[\frac{\text{Mass of solute}}{\text{Mass of solvent in Kg}}\]
Normality (N)
We can define it as the number of equivalents of the solute present in the solution, and it is also called equivalent concentration. The formula for normality is given as follows.
Normality (N) = \[\frac{\text{Weight of solute in grams}}{\text{Equivalent mass} \times \text{Volume in litre}}\]
Mole Fraction:
The mole fraction (X) of a component in a solution is defined as the ratio of the number of moles of that component to the total number of moles of all components in the solution. The mole fraction of A is expressed as XA with the help of the following equation in a solution consisting of A, B, C, … we can calculate XA.
X\[_{A}\] = \[\frac{\text{moles of A}}{\text{mole of A + mole of B + momle of C +.... }}\]
Similarly, we can calculate the mole fraction of B, XB with the help of the following formula.
X\[_{B}\] = \[\frac{\text{moles of B}}{\text{mole of A + mole of B + momle of C +.... }}\]
Now that you know how to find the concentration of a solution using various concentrations of solution formulas, we will try to solve some concentrations of solution questions.
Solved Problems
Question 1) 2 ml of water is added to 4 g of a powdered drug. The final volume is 3ml. Find the mass by volume percentage of the solution?
Answer 1) Given, Mass of solute = 4g
Volume of solution = 3ml
Mass by volume percentage = \[\frac{\text{Mass of solute}}{\text{Volume of solution}}\]x100 = \[\frac{4g}{3ml}\] = 133%
Therefore, the mass by volume percentage is 133 %.
Question 2) Many people use a solution of Na3PO4 to clean walls before putting up wallpaper. The recommended concentration is 1.7 % (m/v). Find the mass of Na3PO4 needed to make 2.0 L of the solution?
Answer 2) Given,
Mass/Volume percentage = 1.7 %
Volume of Solution = 2000 ml
Mass by volume percentage = \[\frac{\text{Mass of solute}}{\text{Volume of solution}}\] × 100
1.7 % = \[\frac{\text{Mass of solute}}{2000ml}\] ×100
Mass of solute = 34 g
Therefore, the mass required is 34 g.
In chemistry, we are often required to calculate the concentration of the solution. The above-mentioned methods of expressing the concentration of a solution are important. The solved examples are helpful for a better understanding of the concept of concentration of a solution.
FAQs on Concentration of Solution
1. What does the concentration of a solution mean in chemistry?
In simple terms, the concentration of a solution tells you how much of a substance (the solute) is dissolved in another substance (the solvent). A solution with a high concentration has a lot of solute, while a solution with a low concentration has very little. Think of it like making sweet tea: a little sugar makes it less concentrated (less sweet), while a lot of sugar makes it highly concentrated (very sweet).
2. What are the most common ways to express the concentration of a solution?
Chemists use several methods to describe concentration precisely, depending on the need. The most common ones include:
- Molarity (M): Moles of solute per litre of solution.
- Molality (m): Moles of solute per kilogram of solvent.
- Mass Percentage (% w/w): The mass of the solute divided by the total mass of the solution, multiplied by 100.
- Volume Percentage (% v/v): The volume of the solute divided by the total volume of the solution, multiplied by 100.
- Normality (N): Gram equivalent of solute per litre of solution.
- Parts Per Million (PPM): Used for very dilute solutions, like measuring pollutants in water.
3. What is the difference between a dilute and a concentrated solution?
A dilute solution contains a relatively small amount of dissolved solute in a large amount of solvent. In contrast, a concentrated solution contains a large amount of dissolved solute for the amount of solvent. For example, light-coloured fruit juice is considered dilute, while dark, strong-tasting juice from a concentrate is considered concentrated. These terms are relative and don't provide an exact measurement.
4. Can you give a real-world example of solution concentration?
A great real-world example is the saline solution used in hospitals. A 'normal' saline solution has a specific concentration of 0.9% mass/volume of sodium chloride (salt) in water. This concentration is crucial because it is isotonic with human blood, meaning it won't damage blood cells. If the concentration were too high or too low, it could be harmful to the patient.
5. What is the main difference between Molarity and Molality?
The key difference is in the denominator of their formulas. Molarity (M) measures the moles of solute per litre of the entire solution (solute + solvent). In contrast, Molality (m) measures the moles of solute per kilogram of the solvent only. This distinction is important because the volume of a solution can change with temperature, but the mass of the solvent does not.
6. Why is molality sometimes preferred over molarity in scientific experiments?
Molality is often preferred in experiments that involve temperature changes because it is temperature-independent. The mass of a solvent does not change when it is heated or cooled. However, the volume of a solution can expand or contract with temperature. This means the molarity of the same solution can be slightly different at different temperatures, which could affect the accuracy of sensitive results.
7. How does changing the temperature affect a solution's concentration?
Changing the temperature can affect concentration units that are based on volume, such as Molarity. When you heat a solution, it typically expands, increasing its volume. Since Molarity = moles/volume, an increase in volume will cause the molarity to decrease. Concentration units based on mass, like Molality or mass percentage, are generally not affected by temperature because mass remains constant.
8. If I keep adding salt to water, will the concentration increase indefinitely?
No, the concentration will not increase forever. At a certain point, the solution will become saturated. This means the solvent (water) cannot dissolve any more solute (salt) at that specific temperature. Once saturation is reached, any extra salt you add will simply sink to the bottom without dissolving, and the concentration of the solution will remain constant.
